[clang-tidy] Support IgnoredRegexp configuration to selectively suppress identifier naming checks

The idea of suppressing naming checks for variables is to support code bases that allow short variables named e.g 'x' and 'i' without prefix/suffixes or casing styles. This was originally proposed as a 'ShortSizeThreshold' however has been made more generic with a regex to suppress identifier naming checks for those that match.

Reviewed By: njames93, aaron.ballman

Differential Revision: https://reviews.llvm.org/D90282