Update private_typeinfo's `is_equal` implementation after r361913

The libc++ typeinfo implementation is being improved to better
handle non-merged type names.

This patch takes advantage of that more correct behavior by delegating
to std::type_infos default operator== instead of doing pointer equality
ourselves.

However, libc++ still expects unique RTTI by default, and so we
should still fall back to strcmp when explicitly requested.

llvm-svn: 361916