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.

try_emplace can default-construct values, so we do not need to do so
on our own.  Plus, try_emplace(Key) is much shorter than
insert(std::make_pair(Key, Value()).

Add:
mapAtomInstance - map the atom group number to a new group.
RemapSourceAtom - apply the mapped atom group number to this instruction.

Modify:
CloneBasicBlock - Call mapAtomInstance on cloned instruction's DebugLocs
if MapAtoms is true (default). Setting to false could
lead to a degraded debugging experience. See code comment.

Optimisations like loop unroll that duplicate instructions need to remap source
atom groups so that each duplicated source construct instance is considered
distinct when determining is_stmt locations.

This commit adds the remapping functionality and a unittest.

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

Summary:
We used the set only to check if it contains certain metadata nodes.
Replacing the set with a predicate makes the intention clearer and the
API more general.

Test Plan:
ninja check-all

Fix assertion failure when building PAuth-hardened code with LTO. W/o assertions we end with invalid codegen.

Summary:
To avoid cloning module-level debug info (owned by the module rather
than the function), CloneFunction implementation used to eagerly
identity map such debug info into ValueMap's MD map. In larger modules
with meaningful volume of debug info this gets very expensive.

By passing such debug info metadata via an IdentityMD set for the
ValueMapper to map on first use, we get several benefits:

1. Mapping metadata is not cheap, particularly because of tracking. When
   cloning a Function we identity map lots of global module-level
   metadata to avoid cloning it, while only a fraction of it is actually
   used by the function. Mapping on first use is a lot faster for
   modules with meaningful amount of debug info.

2. Eagerly identity mapping metadata makes it harder to cache
   module-level data (e.g. a set of metadata nodes in a \a DICompileUnit).
   With this patch we can cache certain module-level metadata
   calculations to speed things up further.

Anecdata from compiling a sample cpp file with full debug info shows that this moderately speeds up
CoroSplitPass which is one of the heavier users of cloning:

|                 | Baseline | IdentityMD set |
|-----------------|----------|----------------|
| CoroSplitPass   | 306ms    | 221ms          |
| CoroCloner      | 101ms    | 72ms           |
| Speed up        | 1x       | 1.4x           |

Test Plan:
ninja check-llvm-unit
ninja check-llvm

If an argument cannot be mapped in `Mapper::mapValue`, it can be mapped
to `poison` instead of `undef`.

It is almost always simpler to use {} instead of std::nullopt to
initialize an empty ArrayRef. This patch changes all occurrences I could
find in LLVM itself. In future the ArrayRef(std::nullopt_t) constructor
could be deprecated or removed.