[Sema] Actually map a variable template specialization from pattern to instantiation

[Sema] Actually map a variable template specialization from pattern to instantiation

We were previously just using a specialization in the class template instead of
creating a new specialization in the class instantiation.

Fixes llvm.org/PR42779.

Differential revision: https://reviews.llvm.org/D65359

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@367367 91177308-0d34-0410-b5e6-96231b3b80d8