[TextAPI] Fix & Add tests for tbd files version 3.

- There was a simple typo in TextStub code that prevented version 3 files to be read.
- Included a version 3 unit test to handle the differences in the format.
- Also a typo in Error.h inside the comments.

https://reviews.llvm.org/D66041

This patch is from Cyndy Ishida <cyndy_ishida@apple.com>.

llvm-svn: 368630