Added formatAndApplyAllReplacements that works on multiple files in libTooling.

Summary:
formatAndApplyAllReplacements takes a set of Replacements, applies them on a
Rewriter, and reformats the changed code.

Reviewers: klimek, djasper

Subscribers: ioeric, klimek, cfe-commits

Differential Revision: http://reviews.llvm.org/D17852