[analyzer] Fix CloneDetector crash on calling methods of class templates.

If a call expression represents a method call of a class template,
and the method itself isn't templated, then the method may be considered
to be a template instantiation without template specialization arguments.

No longer crash when we could not find template specialization arguments.

Patch by Raphael Isemann!

Differential Revision: https://reviews.llvm.org/D23780

llvm-svn: 279529