[libTooling] In Transformer, allow atomic changes to span multiple files.

Summary:
Currently, all changes returned by a single application of a rule must fit in
one atomic change and therefore must apply to one file. However, there are
patterns in which a single rule will want to modify multiple files; for example,
a header and implementation to change a declaration and its definition. This
patch relaxes Transformer, libTooling's interpreter of RewriteRules, to support
multiple changes.

Reviewers: gribozavr

Subscribers: mgrang, jfb, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D80239