[TextAPI] TBD Reader/Writer

Add basic infrastructure for reading and writting TBD files (version 1 - 3).

The TextAPI library is not used by anything yet (besides the unit tests). Tool
support will be added in a separate commit.

The TBD format is currently documented in the implementation file (TextStub.cpp).

https://reviews.llvm.org/D53945

llvm-svn: 347808