[libc++] [NFC] Disable clang-tidy's readability-identifier-naming check

In libc++ most of the names are not conforming to the llvm style. Removing the readability-identifier-naming check removes almost all clang-tidy warnings. For example in `<string>` the warning count goes from 1001 warnings down to 7.

Reviewed By: #libc, Mordante, ldionne

Spies: Mordante, Quuxplusone, aheejin, libcxx-commits, carlosgalvezp

Differential Revision: https://reviews.llvm.org/D113849