[libTooling] Add various Stencil combinators for expressions.

Summary:
This revision adds three new Stencil combinators:
* `expression`, which idiomatically constructs the source for an expression,
  including wrapping the expression's source in parentheses if needed.
* `deref`, which constructs an idiomatic dereferencing expression.
* `addressOf`, which constructs an idiomatic address-taking expression.

Reviewers: gribozavr

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D68315

llvm-svn: 373593