[DebugInfo] Handle types with @_originallyDefinedIn in DebugInfo
When processing a nominal type that has the @_originallyDefinedIn attribute,
IRGenDebugInfo emits a forward declaration of the type as a child
of the original module, and the type with a specification pointing to
the forward declaration. We do this so LLDB has enough information to
both find the type in reflection metadata (the parent module name) and
find it in the swiftmodule (the module name in the type mangled name).
rdar://137146961
