[libTooling] Adds more support for constructing object access expressions.

This patch adds a `buildAccess` function, which constructs a string with the
proper operator to use based on the expression's form and type. It also adds two
predicates related to smart pointers, which are needed by `buildAccess` but are
also of general value.

We deprecate `buildDot` and `buildArrow` in favor of the more general
`buildAccess`. These will be removed in a future patch.

Differential Revision: https://reviews.llvm.org/D116377