Thread safety analysis: Handle compound assignment and ->* overloads

Like regular assignment, compound assignment operators can be assumed to
write to their left-hand side operand. So we strengthen the requirements
there. (Previously only the default read access had been required.)

Just like operator->, operator->* can also be assumed to dereference the
left-hand side argument, so we require read access to the pointee. This
will generate new warnings if the left-hand side has a pt_guarded_by
attribute. This overload is rarely used, but it was trivial to add, so
why not. (Supporting the builtin operator requires changes to the TIL.)

Reviewed By: aaron.ballman

Differential Revision: https://reviews.llvm.org/D124966