[LibTooling] Add Stencil library for format-string style codegen.

Summary:
This file defines the *Stencil* abstraction: a code-generating object, parameterized by named references to (bound) AST nodes.  Given a match result, a stencil can be evaluated to a string of source code.

A stencil is similar in spirit to a format string: it is composed of a series of raw text strings, references to nodes (the parameters) and helper code-generation operations.

See thread on cfe-dev list with subject "[RFC] Easier source-to-source transformations with clang tooling" for background.

Reviewers: sbenza

Reviewed By: sbenza

Subscribers: ilya-biryukov, mgorny, jfb, jdoerfert, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D59371

llvm-svn: 358691