[TextAPI] Update reader to be supported by lib/Object

Summary:
To be able to use the TextAPI/Reader for tbd file consumption (by libObject)
it gets passed a MemoryBufferRef which isn't castable to MemoryBuffer.
Updated the tests to expect that input as well.

Reviewers: ributzka, steven_wu

Reviewed By: steven_wu

Subscribers: hiraditya, dexonsmith, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D66147

llvm-svn: 369119