[libc++] Install llvm-16-dev explicitly until LLVM16 is stable

The clang-tidy checks can't be properly implemented before LLVM16 because the Clang CMake files don't provide the version before.

Reviewed By: Mordante, #libc

Spies: libcxx-commits, arichardson

Differential Revision: https://reviews.llvm.org/D140071