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

[David Blaikie via cfe-commits]

dwblaikie wrote:

Cleaning up some old branches - @pogo59 @rnk who commented on the original https://reviews.llvm.org/D152017

I think the only outstanding thing was the flag name, I've renamed it from `-gincomplete-types` to `-gomit-unreferenced-members` to try to address the feedback. It's not totally precise (it's only member functions that are affected, not member variables, for instance) but seems close enough.

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