[libc++] Enable more clang-tidy checks and list potential candidates

These are some checks that make sense in libc++ IMO. The checks after `#TODO: investigate these checks` are candidates, but they can't be enabled without some cleanup.

Reviewed By: ldionne, #libc

Spies: aheejin, libcxx-commits

Differential Revision: https://reviews.llvm.org/D120925