[libTooling] Add `access` and `ifBound` combinators to Stencil library.

Summary:
This revision add the `access` and `ifBound` combinators to the Stencil library:
* `access` -- constructs an idiomatic expression for accessing a member (a
  `MemberExpr`).
* `ifBound` -- chooses between two `StencilParts` based on the whether an id is
  bound in the match (corresponds to the combinator of the same name in
  RangeSelector).

Reviewers: gribozavr

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D67633

llvm-svn: 372605