[Preprocesssor] Filename should fall back to the written name when typo correction fails.

Summary:
The test is added in  Testcase is at https://reviews.llvm.org/D52775. I tried to add the test to clang's code
completion test, it doesn't reproduce the crash.

Reviewers: sammccall, kristina

Reviewed By: sammccall

Subscribers: kristina, ilya-biryukov, ioeric, MaskRay, jkorous, arphaman, kadircet, cfe-commits

Differential Revision: https://reviews.llvm.org/D52774

llvm-svn: 343592