[clang-tdiy] Add header file extension configuration support.

Summary: * Add a `HeaderFileExtensions` check option in misc-definitions-in-headers, google-build-namespaces and google-global-names-in-headers.

Reviewers: aaron.ballman, alexfh

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D16113

llvm-svn: 259879