[LibTooling] Extend Transformer to support multiple simultaneous changes.

Summary: This revision allows users to specify independent changes to multiple (related) sections of the input.  Previously, only a single section of input could be selected for replacement.

Reviewers: ilya-biryukov

Reviewed By: ilya-biryukov

Subscribers: jfb, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D60408

llvm-svn: 358697