[Clang] [Sema] Handle `this` in `__restrict`-qualified member functions properly (#83187)

When resolving the type of `this` inside a member function, we were
attaching all qualifiers present on the member function to the class
type and then making it a pointer; however, `__restrict`, unlike `const`
and `volatile`, needs to be attached to the pointer type rather than the
pointee type.

This fixes #82941, #42411, and #18121.