Use Rewriter::overwriteChangedFiles() directly

This replaces the custom code in RefactoringTool::saveRewrittenFiles() which
lacked atomic file saving and error diagnostics, resolving an old FIXME from
r157331.

Landing this time with the proper return code, plus a very unhelpful comment
cleared up.

Rubber-stamped by Manuel Klimek.

llvm-svn: 193594