[compiler-rt] [Darwin][CompilerRT] Set compiler ID to 'Clang' for Compiler RT Tests for Apple Clang (PR #121858)

[Saleem Abdulrasool via llvm-commits]

compnerd wrote:

> I'm not entirely sure the PR to not detect as GCC needed to change the previous behavior of setting clang strings to clang.

The point was to try to reduce the special case paths. If there is a specific reason that we cannot treat `AppleClang` as we do the other compiles (i.e. pass through the identity unmodified), that would be good to document.

Doing that in two phases where we first restore the behaviour and then follow up with a change to make it follow the same behaviour is fine too. But the idea is that we should try to bring the configuration into the same path if possible or document why it needs to be different.

https://github.com/llvm/llvm-project/pull/121858