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

(Revised version of a previous, unreviewed, PR.)

Move all expression verification into its only client: DWARFVerifier.
Move all printing code (which was a mix of static and member functions)
into a separate class.

This is one in a series of refactoring PRs to separate dwarf
functionality into lower-level pieces usable without object files and
sections at build time. The code is already written this way via various
"if (section == nullptr)" and similar conditionals. So the functionality
itself is needed and exists, but only as a runtime feature. The goal of
these refactors is to remove the build-time dependencies, which will
allow the existing functionality to be used from lower-level parts of
the compiler. Particularly from lib/MC/.... More information at:


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

This patch transforms:

  X && *X == Y

to:

  X == Y

where X is of std::optional<T>, and Y is of T or similar.

This patch introduces support for the DWARF operation
`DW_OP_GNU_implicit_pointer `(value 0xf3) within LLVM's DWARF parsing
and expression evaluation infrastructure. This GNU extension is used to
describe the location of a variable that is itself a pointer, where the
value of this pointer is stored at an address derived from another DWARF
location expression plus a constant offset.
Motivation:
Compilers like GCC use `DW_OP_GNU_implicit_pointer `to represent the
location of certain variables.Without support for this opcode, debuggers
like LLDB (and other tools relying on LLVM's DWARF libraries) cannot
correctly resolve the location of such variables, leading to an
inability to inspect their values or an incorrect debugging experience.

This allows `llvm-dwarfdump` to decode the DWARF 5 opcode
`DW_OP_implicit_pointer` (0xa0). GCC makes use of this opcode in recent
versions. LLVM contains some (unfinished) support as well. With existing
usage in the ecosystem, adding decoding support here seems reasonable.

This sorts DWARF op descriptions in `DWARFExpression.cpp` by opcode and version, packing the standardised ops together. A few ops also had the wrong version listed, so this fixes those versions as well. (The version does not appear to actually be used currently.)

Fixes #93104

Prevent a crash by only printing DWARFUnit-unaware information in cases
in which `DWARFUnit* U` is `nullptr`.

In preparation for removing the `#include "llvm/ADT/StringExtras.h"`
from the header to source file of `llvm/Support/Error.h`, first add in
all the missing includes that were previously included transitively
through this header.