[clang] Add __is_trivially_equality_comparable

This patch adds a new trait to allow standard libraries to forward `std::equal` calls to `memcmp` in more cases.

Reviewed By: aaron.ballman

Spies: Mordante, shafik, xbolva00, libcxx-commits, cfe-commits, ldionne

Differential Revision: https://reviews.llvm.org/D147175