Revert "Use Rewriter::overwriteChangedFiles() directly"

This wasn't ready for prime time yet, seems to break tools-extra.

This reverts commit r193590.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@193592 91177308-0d34-0410-b5e6-96231b3b80d8