Relands #149701 which was reverted in
https://github.com/llvm/llvm-project/commit/185ae5cdc695248b58ae017508cc764c19bee5b7
because it broke demangling of Itanium symbols on i386.

The last commit in this PR adds the fix for this (discussed in #160930).
On x86 environments, the prefix of `__cdecl` functions will now be
removed to match DWARF. I opened #161676 to discuss this for the other
calling conventions.

This reverts commit e98f34eb08b2bf7aed787e7f8a7cea9111f044c8.

This broke demangling of Itanium symbols on i386.

Before, functions created using the NativePDB plugin would not know
about their mangled name. This showed when printing a stacktrace. There,
only the function name was shown. For
https://github.com/llvm/llvm-project/issues/143149, the mangled function
name is required to separate different parts.

This PR adds that name if available.

The Clang AST nodes also take in a mangled name, which was previously
unset. I don't think this unblocks anything further, because Clang can
mangle the function anyway.

This PR implements `SymbolFileNativePDB::AddSymbols` which adds public
symbols to the symbol table.

These symbols are found in the publics stream. It contains mangled names
coupled with addresses. Addresses are a pair of (segment, offset).
If I understood correctly, then the segment is the section ID from the
COFF header. Sections are already
[constructed](https://github.com/llvm/llvm-project/blob/c48ec7fb60b5e0b4100731d75f82ea63c0ec7b45/lldb/source/Plugins/ObjectFile/PECOFF/ObjectFilePECOFF.cpp#L1048)
using this 1-based index ([MS
docs](https://learn.microsoft.com/en-us/windows/win32/debug/pe-format#section-table-section-headers)).
This allows us to use `section_list->FindSectionByID`.

Lit allows you to set environment variables for all tests in a directory
using a `lit.local.cfg` file. Do this for the PDB and NativePDB tests.

These tests fail if you build without the x86 llvm backend.
Either because they use an x86 triple or try to backtrace which
requires some x86 knowledge to see all frames.

Reviewed By: labath

Differential Revision: https://reviews.llvm.org/D100194

Local values are constants or addresses that can't be folded into
the instruction that uses them. FastISel materializes these in a
"local value" area that always dominates the current insertion
point, to try to avoid materializing these values more than once
(per block).

https://reviews.llvm.org/D43093 added code to sink these local
value instructions to their first use, which has two beneficial
effects. One, it is likely to avoid some unnecessary spills and
reloads; two, it allows us to attach the debug location of the
user to the local value instruction. The latter effect can
improve the debugging experience for debuggers with a "set next
statement" feature, such as the Visual Studio debugger and PS4
debugger, because instructions to set up constants for a given
statement will be associated with the appropriate source line.

There are also some constants (primarily addresses) that could be
produced by no-op casts or GEP instructions; the main difference
from "local value" instructions is that these are values from
separate IR instructions, and therefore could have multiple users
across multiple basic blocks. D43093 avoided sinking these, even
though they were emitted to the same "local value" area as the
other instructions. The patch comment for D43093 states:

  Local values may also be used by no-op casts, which adds the
  register to the RegFixups table. Without reversing the RegFixups
  map direction, we don't have enough information to sink these
  instructions.

This patch undoes most of D43093, and instead flushes the local
value map after(*) every IR instruction, using that instruction's
debug location. This avoids sometimes incorrect locations used
previously, and emits instructions in a more natural order.

In addition, constants materialized due to PHI instructions are
not assigned a debug location immediately; instead, when the
local value map is flushed, if the first local value instruction
has no debug location, it is given the same location as the
first non-local-value-map instruction.  This prevents PHIs
from introducing unattributed instructions, which would either
be implicitly attributed to the location for the preceding IR
instruction, or given line 0 if they are at the beginning of
a machine basic block.  Neither of those consequences is good
for debugging.

This does mean materialized values are not re-used across IR
instruction boundaries; however, only about 5% of those values
were reused in an experimental self-build of clang.

(*) Actually, just prior to the next instruction. It seems like
it would be cleaner the other way, but I was having trouble
getting that to work.

This reapplies commits cf1c774d and dc35368c, and adds the
modification to PHI handling, which should avoid problems
with debugging under gdb.

Differential Revision: https://reviews.llvm.org/D91734

This reverts commit cf1c774d6ace59c5adc9ab71b31e762c1be695b1.

This change caused several regressions in the gdb test suite - at least
a sample of which was due to line zero instructions making breakpoints
un-lined. I think they're worth investigating/understanding more (&
possibly addressing) before moving forward with this change.

Revert "[FastISel] NFC: Clean up unnecessary bookkeeping"
This reverts commit 3fd39d3694d32efa44242c099e923a7f4d982095.

Revert "[FastISel] NFC: Remove obsolete -fast-isel-sink-local-values option"
This reverts commit a474657e30edccd9e175d92bddeefcfa544751b2.

Revert "Remove static function unused after cf1c774."
This reverts commit dc35368ccf17a7dca0874ace7490cc3836fb063f.

Revert "[lldb] Fix TestThreadStepOut.py after "Flush local value map on every instruction""
This reverts commit 53a14a47ee89dadb8798ca8ed19848f33f4551d5.

Local values are constants or addresses that can't be folded into
the instruction that uses them. FastISel materializes these in a
"local value" area that always dominates the current insertion
point, to try to avoid materializing these values more than once
(per block).

https://reviews.llvm.org/D43093 added code to sink these local
value instructions to their first use, which has two beneficial
effects. One, it is likely to avoid some unnecessary spills and
reloads; two, it allows us to attach the debug location of the
user to the local value instruction. The latter effect can
improve the debugging experience for debuggers with a "set next
statement" feature, such as the Visual Studio debugger and PS4
debugger, because instructions to set up constants for a given
statement will be associated with the appropriate source line.

There are also some constants (primarily addresses) that could be
produced by no-op casts or GEP instructions; the main difference
from "local value" instructions is that these are values from
separate IR instructions, and therefore could have multiple users
across multiple basic blocks. D43093 avoided sinking these, even
though they were emitted to the same "local value" area as the
other instructions. The patch comment for D43093 states:

  Local values may also be used by no-op casts, which adds the
  register to the RegFixups table. Without reversing the RegFixups
  map direction, we don't have enough information to sink these
  instructions.

This patch undoes most of D43093, and instead flushes the local
value map after(*) every IR instruction, using that instruction's
debug location. This avoids sometimes incorrect locations used
previously, and emits instructions in a more natural order.

This does mean materialized values are not re-used across IR
instruction boundaries; however, only about 5% of those values
were reused in an experimental self-build of clang.

(*) Actually, just prior to the next instruction. It seems like
it would be cleaner the other way, but I was having trouble
getting that to work.

Differential Revision: https://reviews.llvm.org/D91734

This reverts commit 73a6a164b84a8195defbb8f5eeb6faecfc478ad4.