Fix crash-on-invalid bug in template instantiation.

Get rid of code-path that (according to Richard Smith) is not needed but
leads to a crasher bug when assuming a template has been fully
instantiated and thus has a definition.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@240752 91177308-0d34-0410-b5e6-96231b3b80d8