Generalize future keyword compat diagnostics.

This, in preparation for the introduction of more new keywords in the
implementation of the C++ language, generalizes the support for future keyword
compat diagnostics (e.g., diag::warn_cxx11_keyword) by extending the
applicability of the relevant property in IdentifierTable with appropriate
renaming.

Patch by Hubert Tong!

llvm-svn: 237332