Fix import of class templates partial specialization

Summary:
Currently there are several issues with the import of class template
specializations.  (1) Different TUs may have class template specializations
with the same template arguments, but with different set of instantiated
MethodDecls and FieldDecls.  In this patch we provide a fix to merge these
methods and fields.  (2) Currently, we search the partial template
specializations in the set of simple specializations and we add partial
specializations as simple specializations. This is bad, this patch fixes it.

Reviewers: a_sidorin, xazax.hun, r.stahl

Subscribers: rnkovacs, dkrupp, cfe-commits

Differential Revision: https://reviews.llvm.org/D50451

llvm-svn: 340402