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

[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

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@345699 91177308-0d34-0410-b5e6-96231b3b80d8