[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