[clang] Add option to avoid generating coverage mappings for unused functions (PR #92582)

[Zequan Wu via cfe-commits]

ZequanWu wrote:

I think the main purpose for emitting coverage mapping for un-emitted functions is to tell llvm-cov that those functions are not executed at all, so it shows execution count 0 for them instead of not-covered.

https://github.com/llvm/llvm-project/pull/92582