Fetch the DILocation once, instead of many times. This is pretty
trivial, but goes through out-of-line code.

Patch 3/4 adding bitcode support, though the final patch doesn't depend on this
one.

Prior to this patch, a Key Instructions function inlined into a
Not-Key-Instructions function fell back to Not-Key-Instructions handling.

In order to fully support inlining mixed modes we need to run
`computeKeyInstructions` (in case there's a Key Instructions scope) and
`findForceIsStmtInstrs` (in case there's a Not-Key-Instructions scope) on all
functions. This has a slight performance cost for all configurations - see PR
for details.

Patch 2/4 adding bitcode support.

A non-key-instructions function inlined into a key-instructions function uses
non-key-instructions is_stmt placement (without `findForceIsStmtInstrs`).

A key-instructions function inlined into a non-key-instructions function
currently results in falling back to non-key-instructions for the inlined scope
too.

Both of these concessions (not using `findForceIsStmtInstrs` in the 1st case,
and not using Key Instructions for the inlined scope in the 2nd) are for
performance reasons; to do the right thing we'd need to run both
`findForceIsStmtInstrs` and `computeKeyInstructions` - in case that's
controversial I've got a separate PR for that: PR 144103.

Reapply "[NFC][DebugInfo][DWARF] Create new low-level dwarf library (#…
(#145959)
    
This reapplies cbf781f0bdf2f680abbe784faedeefd6f84c246e, with fixes for
the shared-library build and the unconventional sanitizer-runtime build.

Original Description:

This is the culmination of a series of changes described in [1].
    
Although somewhat large by line count, it is almost entirely mechanical,
creating a new library in DebugInfo/DWARF/LowLevel. This new library has
very minimal dependencies, allowing it to be used from more places than
the normal DebugInfo/DWARF library--in particular from MC.
    
1.
https://discourse.llvm.org/t/rfc-debuginfo-dwarf-refactor-into-to-lower-and-higher-level-libraries/86665/2

…145081)"

This reverts commit cbf781f0bdf2f680abbe784faedeefd6f84c246e.

Breaks a couple of buildbots.

This is the culmination of a series of changes described in [1].
    
Although somewhat large by line count, it is almost entirely mechanical,
creating a new library in DebugInfo/DWARF/LowLevel. This new library has
very minimal dependencies, allowing it to be used from more places than
the normal DebugInfo/DWARF library--in particular from MC.
    
I am happy to put it in another location, or to structure it differently
if that makes sense. Some have suggested in BinaryFormat, but it is not
a great fit there. But if that makes more sense to the reviewers, I can
do that.
 
Another possibility would be to use pass-through headers to allow
clients who don't care to depend only on DebugInfo/DWARF. This would be
a much less invasive change, and perhaps easier for clients. But also a
system that hides details.

Either way, I'm open.

1.
https://discourse.llvm.org/t/rfc-debuginfo-dwarf-refactor-into-to-lower-and-higher-level-libraries/86665/2

- try_emplace(Key) is shorter than insert(std::make_pair(Key, 0)).
- try_emplace performs value initialization without value parameters.
- We overwrite values on successful insertion anyway.

These are identified by misc-include-cleaner.  I've filtered out those
that break builds.  Also, I'm staying away from llvm-config.h,
config.h, and Compiler.h, which likely cause platform- or
compiler-specific build failures.

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