[clang-tidy] readability-identifier-naming checks configs for included files

When checking for the style of a decl that isn't in the main file, the check will now search for the configuration that the included files uses to gather the style for its decls.

This can be useful to silence warnings in header files that follow a different naming convention without using header-filter to silence all warnings(even from other checks) in the header file.

Reviewed By: aaron.ballman, gribozavr2

Differential Revision: https://reviews.llvm.org/D84814