[ASTImporter] Fix ClassTemplateSpecialization in wrong DC

Summary:
ClassTemplateSpecialization is put in the wrong DeclContex if implicitly
instantiated. This patch fixes it.

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

Subscribers: rnkovacs, dkrupp, cfe-commits

Differential Revision: https://reviews.llvm.org/D47058

llvm-svn: 333269