[libTooling] Add stencil combinators for nodes that may be pointers or values.

Summary:
Adds combinators `maybeDeref` and `maybeAddressOf` to provide a uniform way to handle
nodes which may be bound to either a pointer or a value (most often in the
context of member expressions). Such polymorphism is already supported by
`access`; these combinators extend it to more general uses.

Reviewers: gribozavr

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D70554