Fix PR19169 [Crash on invalid attempting to specialize a template method as a template variable].

A template declaration of a template name can be null in case we have a dependent name or a set of function templates.
Hence use dyn_cast_or_null instead of dyn_cast. Also improve the diagnostic emitted in this case.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@208313 91177308-0d34-0410-b5e6-96231b3b80d8