[clang-tidy] Fix template agrument false positives in unused-using-decls.

Summary:
* Fix a false postive when an using class is used in an explicit template instantiation.
* Fix a false postive when an using template class is used as template argument.

Reviewers: aaron.ballman

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D25437

llvm-svn: 283879