[LibTooling] Extend `RewriteRule` with support for adding includes.

Summary:
This revision allows users to specify the insertion of an included directive (at
the top of the file being rewritten) as part of a rewrite rule.  These
directives are bundled with `RewriteRule` cases, so that different cases can
potentially result in different include actions.

Reviewers: ilya-biryukov, gribozavr

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D63892

llvm-svn: 364917