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
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@347808 91177308-0d34-0410-b5e6-96231b3b80d8
