[libcxx] [llvm] [libcxx][llvm]: Fix misidentification of GNU-style clang as clang-cl (PR #225124)

[Jacob Abel via llvm-commits]

OneDeuxTriSeiGo wrote:

I split up the PR into two commits so that they can be consumed by downstreams easier in the meantime.

https://github.com/llvm/llvm-project/pull/225124