Fix warning in builds without asserts.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@188132 91177308-0d34-0410-b5e6-96231b3b80d8
Avoid spurious error messages if parent template class cannot be instantiated

Differential Revision: http://llvm-reviews.chandlerc.com/D924


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@188133 91177308-0d34-0410-b5e6-96231b3b80d8