[clang] [clang] Restrict -Wnrvo to C++ code only. (PR #157059)

[via cfe-commits]

https://github.com/Sirraide commented:

I think it makes sense not to issue the warning considering that C doesn’t have a concept of NRVO from what I know. That said, I don’t think the current approach is right because it just prevents NRVO in C entirely; we still want to perform NRVO, we just don’t want to warn if we can’t.

What I think we should do instead is the same thing that GCC does, i.e. reject the flag in the driver if we’re compiling C.

https://github.com/llvm/llvm-project/pull/157059