[clang] [llvm] [Clang][AIX] Add -mloadtime-comment-vars flag to preserve identifying variables (PR #187986)

[Tony Varghese via cfe-commits]

tonykuttai wrote:

`-mloadtime-comment-vars=` now matches against the variable's mangled IR symbol name — the same name shown by `nm` and `clang -S -emit-llvm`.

For C (the primary use case), file-scope statics are not mangled, so the user experience is unchanged: `-mloadtime-comment-vars=sccsid,version` works exactly as before.

For C++, users pass the Itanium ABI mangled name.

@w2yehia @hubert-reinterpretcast 

https://github.com/llvm/llvm-project/pull/187986