Modules: Adopt template parameters for variable templates to set their decl context correctly

Exposed by a related bug about visibility of default arguments of nested
templates - without the correct decl context, default template
parameters of variable templates nested in classes would have incorrect
visibility computed.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@358796 91177308-0d34-0410-b5e6-96231b3b80d8