[ASTReader] Add test for previous change r306583 / 145692e.

Summary:
Add a test for the change to ASTReader that reproduces the
logic for consolidating multiple ObjC interface definitions to the
case of multiple ObjC protocol definitions.

This test is a modified copy of the test that accompanied the original
change to interfaces, in 2ba1979.

Reviewers: bruno

Reviewed By: bruno

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D34788

llvm-svn: 306732