[stubtest] Allow runtime-existing aliases of types marked as @type_check_only

[brianschubert]

In typeshed, there's a few cases of stubs like this:

class _DoesNotExist: ...  # does not exist at runtime

if sys.version_info >= (3, X):
    Exists = _DoesNotExist

Ideally, it would be nice to mark _DoesNotExit as @type_check_only to make it clear that this type isn't available at runtime. However, this currently can't be done, because doing so will make stubtest think that Exists is also @type_check_only, which sets off alarm bells due to Exists being available at runtime.

This PR makes it so stubtest doesn't consider @type_check_only-status when checking type alias targets, making it possible to mark types like the above as @type_check_only.