[clang-tools-extra] [clang-tidy][libc] Fix namespace check with macro (PR #68134)

[Guillaume Chatelet via cfe-commits]

gchatelet wrote:

It would be ideal to move the [following lines](
https://github.com/llvm/llvm-project/blob/b3c710beda5b45dc18a5cbd74e141c1169971450/clang-tools-extra/clang-tidy/llvmlibc/ImplementationInNamespaceCheck.cpp#L17-L18)  to a shared header and use them in both implementations.

https://github.com/llvm/llvm-project/pull/68134