[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