[libc++] Add clang-tidy to the list of possible substitutions for %{clang-tidy}

Most people don't have a versioned clang-tidy locally. This allows them to still run the clang-tidy test if they have a suitable version.

Reviewed By: ldionne, #libc

Spies: libcxx-commits, arichardson

Differential Revision: https://reviews.llvm.org/D141241