[clang-cl] Inherit dllexport to static locals also in template instantiations (PR39496)

In the course of D51340, @takuto.ikuta discovered that Clang fails to put
dllexport/import attributes on static locals during template instantiation.

For regular functions, this happens in Sema::FinalizeDeclaration(), however for
template instantiations we need to do something in or around
TemplateDeclInstantiator::VisitVarDecl(). This patch does that, and extracts
the code to a utility function.

Differential Revision: https://reviews.llvm.org/D53870

llvm-svn: 345699