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

[Tony Varghese via llvm-commits]

tonykuttai wrote:

> > @tonykuttai, I think the treatment of C++20 modules requires documentation/testing.
> > For example, the option applies to a variable defined in a module unit at the time the module unit is compiled to object code. What happens to specializations of static data members instantiated from imported template definitions? Did we really need to allow static data members?
> 
> Support is now limited to file and namespace-scope variables. All name-matched static data members, variable template specializations (explicit ones included), and function-local statics now get the not-preserved diagnostic. Implicit instantiations are diagnosed via the template-instantiation path, once per instantiating TU.

On testing we found that an unreferenced internal-linkage named variable (the `static char sccsid[]` idiom itself) was silently dropped whenever its definition crossed a serialization boundary. External and module-linkage variables were unaffected. Two causes: `ASTContext::DeclMustBeEmitted` did not know the implicit attribute, and for named modules only non-discardable variables are recorded in the module-initializer lists CodeGen walks when compiling a module unit from its BMI. This is now fixed. 

One deliberate behavior change: since DeclMustBeEmitted now sees the attribute, -Wunused-const-variable no longer fires for a preserved variable.

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