[clang][dataflow] Produce pointer values for callees of member operator calls.

Calls to member operators are a special case in that their callees have pointer
type. The callees of non-operator non-static member functions are not pointers.
See the comments in the code for details.

This issue came up in the Crubit nullability check; the fact that we weren't
modeling the `PointerValue` caused non-convergence.

Reviewed By: ymandel, xazax.hun

Differential Revision: https://reviews.llvm.org/D158592