[ASTImporter] Add new tests about templated-described swing

Summary:
Add a new test about importing a partial specialization (of a class).  Also,
this patch adds new tests about the templated-described swing, some of these
fail ATM, but subsequent patches will fix them.

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

Subscribers: rnkovacs, dkrupp, cfe-commits

Differential Revision: https://reviews.llvm.org/D47534

llvm-svn: 335455