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

[Andrew Wock via cfe-commits]

ajwock wrote:

The problem that this solves is related to binary size.  In situations where a large C API is in generated code but a small part of the API is used, the size of __llvm_prf_names and __llvm_cov_fun sections can be prohibitively large.  This flag offers a way to reduce binary size in this case assuming you aren't concerned with coverage of non-emitted code.

Would this flag make more sense if it were named differently?  For example, `-fcoverage-mapping-only-emitted-code`?

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