[libc++][test] Run clang-tidy during CI

I'm trying to get libc++ to the point of being able to run clang-tidy. This is a PR to see if clang-tidy is happy with all the CI configs.

Reviewed By: Quuxplusone, ldionne, #libc

Spies: mgorny, aheejin, libcxx-commits, arichardson

Differential Revision: https://reviews.llvm.org/D117174