Interpret Key Instructions metadata to determine is_stmt placement.

The lowest rank (highest precedent) instructions in each {InlinedAt, atomGroup}
set are candidates for is_stmt. Only the last instruction in each set in a given
block gets is_stmt. Calls always get is_stmt.

RFC: https://discourse.llvm.org/t/rfc-improving-is-stmt-placement-for-better-interactive-debugging/82668

Reland https://github.com/llvm/llvm-project/pull/106230

The original PR was reverted due to compilation time regression.

This PR fixed that by adding a condition OutStreamer->isVerboseAsm() to
the generation of extra inlined-at debug info, so that it does not
affect normal compilation time.

Currently MC print source location of instructions in comments in
assembly when debug info is available, however, it does not include
inlined-at locations when a function is inlined.

For example, function foo is defined in header file a.h and is called
multiple times in b.cpp. If foo is inlined, current assembly will only
show its instructions with their line numbers in a.h. With inlined-at
locations, the assembly will also show where foo is called in b.cpp.

This patch adds inlined-at locations to the comments by using
DebugLoc::print. It makes the printed source location info consistent
with those printed by machine passes.

This reverts commit f3dc358953a13caf7521fc615a08f6317930351c.

This causes a large compile-time regression:
https://llvm-compile-time-tracker.com/compare.php?from=267403442264959f6b06e227ff450c385f4b3ef2&to=f3dc358953a13caf7521fc615a08f6317930351c&stat=instructions:u

Currently MC print source location of instructions in comments in
assembly when debug info is available, however, it does not include
inlined-at locations when a function is inlined.

For example, function foo is defined in header file a.h and is called
multiple times in b.cpp. If foo is inlined, current assembly will only
show its instructions with their line numbers in a.h. With inlined-at
locations, the assembly will also show where foo is called in b.cpp.

This patch adds inlined-at locations to the comments by using
DebugLoc::print. It makes the printed source location info consistent
with those printed by machine passes.

**Summary**

This patch introduces a new compiler option `-mllvm
-emit-func-debug-line-table-offsets` that enables the emission of
per-function line table offsets and end sequences in DWARF debug
information. This enhancement allows tools and debuggers to accurately
attribute line number information to their corresponding functions, even
in scenarios where functions are merged or share the same address space
due to optimizations like Identical Code Folding (ICF) in the linker.

**Background**
RFC: [New DWARF Attribute for Symbolication of Merged
Functions](https://discourse.llvm.org/t/rfc-new-dwarf-attribute-for-symbolication-of-merged-functions/79434)

Previous similar PR:
[#93137](https://github.com/llvm/llvm-project/pull/93137) – This PR was
very similar to the current one but at the time, the assembler had no
support for emitting labels within the line table. That support was
added in PR [#99710](https://github.com/llvm/llvm-project/pull/99710) -
and in this PR we use some of the support added in the assembler PR.

In the current implementation, Clang generates line information in the
`debug_line` section without directly associating line entries with
their originating `DW_TAG_subprogram` DIEs. This can lead to issues when
post-compilation optimizations merge functions, resulting in overlapping
address ranges and ambiguous line information.

For example, when functions are merged by ICF in LLD, multiple functions
may end up sharing the same address range. Without explicit linkage
between functions and their line entries, tools cannot accurately
attribute line information to the correct function, adversely affecting
debugging and call stack resolution.


**Implementation Details**
To address the above issue, the patch makes the following key changes:

**`DW_AT_LLVM_stmt_sequence` Attribute**: Introduces a new LLVM-specific
attribute `DW_AT_LLVM_stmt_sequence` to each `DW_TAG_subprogram` DIE.
This attribute holds a label pointing to the offset in the line table
where the function's line entries begin.

**End-of-Sequence Markers**: Emits an explicit DW_LNE_end_sequence after
each function's line entries in the line table. This marks the end of
the line information for that function, ensuring that line entries are
correctly delimited.

**Assembler and Streamer Modifications**: Modifies the MCStreamer and
related classes to support emitting the necessary labels and tracking
the current function's line entries. A new flag
GenerateFuncLineTableOffsets is added to control this behavior.

**Compiler Option**: Introduces the `-mllvm
-emit-func-debug-line-table-offsets` option to enable this
functionality, allowing users to opt-in as needed.

This patch follows on from the changes made in #105524, by adding an
additional heuristic that prevents us from applying the start-of-MBB
is_stmt flag when we can see that, for all direct branches to the MBB,
the last line stepped on before the branch is the same as the first line
of the MBB. This is mainly to prevent certain pathological cases, such
as macros that expand to multiple basic blocks that all have the same
source location, from giving us repeated steps on the same line. This
approach is not comprehensive, since it relies on analyzeBranch to read
edges, but the default fallback of applying is_stmt may lead only to
useless steps in some cases, rather than skipping useful steps
altogether.

Enable .debug_loc section for NVPTX backend.

This commit makes NVPTX omit DW_AT_low_pc (and DW_AT_high_pc) for
DW_TAG_compile_unit. This is because cuda-gdb uses the compile unit's
low_pc as a base address, and adds the addresses in the debug_loc
section to it. Removing low_pc is equivalent to setting that base
address to zero, so addition doesn't break the location ranges.
Additionally, this patch forces debug_loc label emission to emit single
labels with no subtraction or base. This would not be necessary if we
could emit `label1 - label2` expressions in PTX. The PTX documentation
at
https://docs.nvidia.com/cuda/parallel-thread-execution/index.html#debugging-directives-section
makes it seem like this is supported, but it doesn't actually work. I
believe when that documentation says that you can subtract “label
addresses between labels in the same dwarf section”, it doesn't merely
mean that the labels need to be in the same section as each other, but
in fact they need to be in the same section as the use. If support for
label subtraction is supported such that in the debug_loc section you
can subtract labels from the main code section, then we can remove the
workarounds added in this PR.

Also, since this now emits valid .debug_loc sections, it replaces the
empty .debug_loc to force existence of at least one debug section with
an empty .debug_macinfo section, which matches what nvcc does.

Currently, we identify the end of the prologue as being "the instruction
that first has *this* DebugLoc". It works well enough, but I feel
identifying a position in a function is best communicated by a
MachineInstr. Plus, I've got some patches coming that depend upon this.

This patch makes LLVM emit line 0 source locations for nops emitted as
basic block padding.

---------

Co-authored-by: Orlando Cazalet-Hyams <orlando.hyams@sony.com>

Some of SIE's post-mortem analysis infrastructure currently makes use of
.debug_aranges, so we'd like to ensure the section's presence in
PlayStation binaries. The simplest way to do this is to force emission
when the debugger tuning is set to SCE (which is in turn typically
initialized from the target triple). This also simplifies the driver.

llvm/test/DebugInfo/debuglineinfo-path.ll has been marked as UNSUPPORTED
on PlayStation. When aranges are emitted, the DWARF in the test case is
such that relocations need to be applied to the aranges section in order
for symbolization to work. An alternative approach would be to implement
the application of relocations in DWARFDebugArangeSet. While experiments
show that this can be made to work with a modest patch, the test cases
would be rather contrived. Since I expect the only utility for such a
change would be to make this test case pass for PlayStation targets, and
few - if any - outside of PlayStation care about aranges, UNSUPPORTED
would seem to be a more practical option.

This was originally commited as 22eb290a96 (#99629) and later reverted
at 84658fb82b (#99711) due to test failures on SIE built bots. These
failures shouldn't recur due to 3b24e5d450 (#99897) and the
aforementioned change to debuglineinfo-path.ll.

SIE tracker: TOOLCHAIN-16951