Implement #pragma clang final extension

This patch adds a new preprocessor extension ``#pragma clang final``
which enables warning on undefinition and re-definition of macros.

The intent of this warning is to extend beyond ``-Wmacro-redefined`` to
warn against any and all alterations to macros that are marked `final`.

This warning is part of the ``-Wpedantic-macros`` diagnostics group.

Reviewed By: aaron.ballman

Differential Revision: https://reviews.llvm.org/D108567