[ASTImporter] Fix missing implict CXXRecordDecl in ClassTemplateSpecializationDecl

Summary:
Currently we do not import the implicit CXXRecordDecl of a
ClassTemplateSpecializationDecl. This patch fixes it.

Reviewers: a.sidorin, xazax.hun, r.stahl

Subscribers: rnkovacs, dkrupp, cfe-commits

Differential Revision: https://reviews.llvm.org/D47057

llvm-svn: 333086