Thread safety analysis: Support builtin pointer-to-member operators

We consider an access to x.*pm as access of the same kind into x, and
an access to px->*pm as access of the same kind into *px. Previously we
missed reads and writes in the .* case, and operations to the pointed-to
data for ->* (we didn't miss accesses to the pointer itself, because
that requires an LValueToRValue cast that we treat independently).

We added support for overloaded operator->* in D124966.

Reviewed By: aaron.ballman

Differential Revision: https://reviews.llvm.org/D129514