Fix false negative in -Wthread-safety-attributes

The original implementation didn't fire on non-template classes when a
base class was an instantiation of a template with a dependent base.
In that case the base of the base is dependent as seen from the base,
but not from the class we're interested in, which isn't a template.

Also it simplifies the code a lot.

Reviewed By: aaron.ballman

Differential Revision: https://reviews.llvm.org/D98724