Use Rewriter::overwriteChangedFiles() directly

The old code in RefactoringTool::saveRewrittenFiles() lacked
atomic moves and diagnostics and had FIXME to this effect.

Rubber-stamped by Manuel Klimek.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@193590 91177308-0d34-0410-b5e6-96231b3b80d8
Remove obsolete RewriteBuffer comment

The string buffer in question was replaced with RewriteRope in r43884
back in 2007.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@193591 91177308-0d34-0410-b5e6-96231b3b80d8