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

[Tony Varghese via cfe-commits]

tonykuttai wrote:

Variables named in `-mloadtime-comment-vars=` are now validated and a warning is emitted when one can't be honored, instead of silently preserving it (and, for a dynamically initialized variable, embedding an empty/wrong value):
- volatile-qualified 
- non-static storage duration (e.g. thread_local)
- dynamically initialized.Constant initialization is now required
- a pointer not bound to a string literal (`const char *p = arr;`). the pointer form must point directly at a string literal.
     - `static const char *p = some_array;` is now diagnosed rather than preserved.  
- const char[] is accepted.
- Wrong type / missing initializer / undefined / function-local static remain silently ignored. 

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