[clang] Add -Wsuggest-override

This patch adds `-Wsuggest-override`, which allows for more aggressive enforcement of modern C++ best practices, as well as better compatibility with gcc, which has had its own `-Wsuggest-override` since version 5.1.

Clang already has `-Winconsistent-missing-override`, which only warns in the case where there is at least one function already marked `override` in a class. This warning strengthens that warning by suggesting the `override` keyword regardless of whether it is already present anywhere.

The text between suggest-override and inconsistent-missing-override is now shared, using `TextSubstitution` for the entire diagnostic text.

Reviewed By: dblaikie

Differential Revision: https://reviews.llvm.org/D82728