[clang-tidy] Expand modular headers for PPCallbacks

Summary:
Add a way to expand modular headers for PPCallbacks. Checks can opt-in for this
expansion by overriding the new registerPPCallbacks virtual method and
registering their PPCallbacks in the preprocessor created for this specific
purpose.

Use module expansion in the readability-identifier-naming check

Reviewers: gribozavr, usaxena95, sammccall

Reviewed By: gribozavr

Subscribers: nemanjai, mgorny, xazax.hun, kbarton, jdoerfert, cfe-commits

Tags: #clang, #clang-tools-extra

Differential Revision: https://reviews.llvm.org/D59528

llvm-svn: 356750