Fix for combined loop and nullptr convert tests

The rewriter was previously reading the content buffer from the file itself.
Since we are now keeping the content in memory and writing to the file only
once, the rewriter's buffer (from the file) was not in sync with the
RefactoringTool's buffer. Adding an overrideFileContents call (similar to how
Clang-format handles for this) will resolve this issue.

Author: Jack Yang <jack.yang@intel.com>
Reviewers: gribozavr, klimek
llvm-svn: 174643