This is an attempt to reland
https://github.com/llvm/llvm-project/pull/159104 with the fix for
https://github.com/llvm/llvm-project/issues/160197.

The original patch had the following problem: when an abstract
subprogram DIE is constructed from within
`DwarfDebug::endFunctionImpl()`,
`DwarfDebug::constructAbstractSubprogramScopeDIE()` acknowledges `unit:`
field of DISubprogram. But an abstract subprogram DIE constructed from
`DwarfDebug::beginModule()` was put in the same compile unit to which
global variable referencing the subprogram belonged, regardless of
subprogram's `unit:`.

This is fixed by adding `DwarfDebug::getOrCreateAbstractSubprogramCU()`
used by both`DwarfDebug:: constructAbstractSubprogramScopeDIE()` and
`DwarfCompileUnit::getOrCreateSubprogramDIE()` when abstract subprogram
is queried during the creation of DIEs for globals in
`DwarfDebug::beginModule()`.

The fix and the already-reviewed code from
https://github.com/llvm/llvm-project/pull/159104 are two separate
commits in this PR.

=====
The original commit message follows:

With this change, construction of abstract subprogram DIEs is split in
two stages/functions: creation of DIE (in
DwarfCompileUnit::getOrCreateAbstractSubprogramDIE) and its population
with children (in
DwarfCompileUnit::constructAbstractSubprogramScopeDIE).

With that, abstract subprograms can be created/referenced from
DwarfDebug::beginModule, which should solve the issue with static local
variables DIE creation of inlined functons with optimized-out
definitions. It fixes https://github.com/llvm/llvm-project/issues/29985.

LexicalScopes class now stores mapping from DISubprograms to their
corresponding llvm::Function's. It is supposed to be built before
processing of each function (so, now LexicalScopes class has a method
for "module initialization" alongside the method for "function
initialization"). It is used by DwarfCompileUnit to determine whether a
DISubprogram needs an abstract DIE before DwarfDebug::beginFunction is
invoked.

DwarfCompileUnit::getOrCreateSubprogramDIE method is added, which can
create an abstract or a concrete DIE for a subprogram. It accepts
llvm::Function* argument to determine whether a concrete DIE must be
created.

This is a temporary fix for
https://github.com/llvm/llvm-project/issues/29985. Ideally, it will be
fixed by moving global variables and types emission to
DwarfDebug::endModule (https://reviews.llvm.org/D144007,
https://reviews.llvm.org/D144005).

Some code proposed by Ellis Hoag <ellis.sparky.hoag@gmail.com> in
https://github.com/llvm/llvm-project/pull/90523 was taken for this
commit.

Reverts llvm/llvm-project#159104 due to the issues reported in
https://github.com/llvm/llvm-project/issues/160197.

With this change, construction of abstract subprogram DIEs is split in
two stages/functions:
creation of DIE (in DwarfCompileUnit::getOrCreateAbstractSubprogramDIE)
and its population with children (in
DwarfCompileUnit::constructAbstractSubprogramScopeDIE).
With that, abstract subprograms can be created/referenced from
DwarfDebug::beginModule, which should solve the issue with static local
variables DIE creation of inlined functons with optimized-out
definitions. It fixes https://github.com/llvm/llvm-project/issues/29985.

LexicalScopes class now stores mapping from DISubprograms to their
corresponding llvm::Function's. It is supposed to be built before
processing of each function (so, now LexicalScopes class has a method
for "module initialization" alongside the method for "function
initialization"). It is used by DwarfCompileUnit to determine whether a
DISubprogram needs an abstract DIE before DwarfDebug::beginFunction is
invoked.

DwarfCompileUnit::getOrCreateSubprogramDIE method is added, which can
create an abstract or a concrete DIE for a subprogram. It accepts
llvm::Function* argument to determine whether a concrete DIE must be
created.

This is a temporary fix for
https://github.com/llvm/llvm-project/issues/29985. Ideally, it will be
fixed by moving global variables and types emission to
DwarfDebug::endModule (https://reviews.llvm.org/D144007,
https://reviews.llvm.org/D144005).

Some code proposed by Ellis Hoag <ellis.sparky.hoag@gmail.com> in
https://github.com/llvm/llvm-project/pull/90523 was taken for this
commit.

This changes the final stage of InstrRef, i.e. the TransferTracker
(which combines the values locations with the variable values), so that
it treats a DEBUG_VALUE of an EntryValue just like a DEBUG_VALUE of a
constant: a location that is never clobbered and can be propagated to
subsequent BBs as long as no other DEBUG_VALUE intrinsics updated the
variable.

We add two tests here:

1. `entry_value_clobbered_stack_copy` that saves a register on the
stack, uses this register as an entry value DBG_VALUE location, and then
clobbers it. Prior to this patch, this test would crash because we would
try to describe a new location for the variable in terms of what was
saved on the stack, and use an invalid expression to do so. This is not
needed as an EntryValue can never be clobbered.

2. `entry_value_gets_propagated`, that tests that an EntryValue
DBG_VALUE is propagated in a diamond-shaped CFG.

This patch is trying to reland
https://github.com/llvm/llvm-project/pull/77938 but also fixes the bug
with InstrRef based LiveDebugValues, where entry values were not being
propagated in a diamond-shaped CFG.

try_emplace can default-construct values, so we do not need to do so
on our own.  Plus, try_emplace(Key) is much simpler/shorter than
insert({Key, LongValueType()}).

This is a reland of #138434 except that:

- the bits for llvm/lib/CodeGen/RenameIndependentSubregs.cpp
  have been dropped because they caused a test failure under asan, and

- the bits for llvm/lib/CodeGen/SelectionDAG/ScheduleDAGFast.cpp have
  been improved with structured bindings.

This reverts commit a9699a334bc9666570418a3bed9520bcdc21518b.

Breaks CodeGen/AMDGPU/collapse-endcf.ll in several configs
(sanitizer builds; macOS; possibly more), see comments on
https://github.com/llvm/llvm-project/pull/138434