[clang] [DebugInfo] Add flag to only emit referenced member functions (PR #87018)

[Stephen Tozer via cfe-commits]

https://github.com/SLTozer approved this pull request.

> Yeah, seems I'm outvoted here. I'm a bit of a pedant for the C++ standard language, which doesn't talk about "methods", only "member functions".

All I'd say is that if we went with members, it ought to be `-gomit-unreferenced-member-functions` rather than `-gomit-unreferenced-members`; I'm not strongly attached to "methods" over "member functions", but just "members" is misleading imo. In any case, the patch functionally LGTM.

https://github.com/llvm/llvm-project/pull/87018