[ASTImporter] Corrected lookup at import of templated record decl

Summary:
When a CXXRecordDecl under ClassTemplateDecl is imported, check
the templated record decl for similarity instead of the template.

Reviewers: a.sidorin

Reviewed By: a.sidorin

Subscribers: martong, cfe-commits

Differential Revision: https://reviews.llvm.org/D47313

Patch by Balazs Keri!

llvm-svn: 333522