[NFC] Add 'override' keyword where missing in include/ and lib/.

This fixes warnings raised by Clang's new -Wsuggest-override, in preparation for enabling that warning in the LLVM build. This patch also removes the virtual keyword where redundant, but only in places where doing so improves consistency within a given file. It also removes a couple unnecessary virtual destructor declarations in derived classes where the destructor inherited from the base class is already virtual.

Differential Revision: https://reviews.llvm.org/D83709