[index] Add the "SpecializationOf" relation to the forward declarations

of class template specializations

This commit fixes an issue where a forward declaration of a class template
specialization was not related to the base template. We need to relate even
forward declarations because specializations don't have to be defined.

rdar://32869409

Differential Revision: https://reviews.llvm.org/D34462

llvm-svn: 305996