[AST] Add structural eq tests for template args

Summary:
New tests added to verify equivalency of templates when their
parameters are different.

Reviewers: a_sidorin, shafik

Subscribers: rnkovacs, dkrupp, Szelethus, gamesh411, cfe-commits

Differential Revision: https://reviews.llvm.org/D57235

llvm-svn: 352345