Debug Info (-gmodules): emit full types for non-anchored template specializations

Before this patch, clang would emit a (module-)forward declaration for
template instantiations that are not anchored by an explicit template
instantiation, but still are guaranteed to be available in an imported
module. Unfortunately detecting the owning module doesn't reliably
work when local submodule visibility is enabled and the template is
inside a cross-module namespace.

This make clang debuggable again with -gmodules and LSV enabled.

rdar://problem/41552377

llvm-svn: 345109