[Modules] Improve error message when cannot find parent module for submodule definition.

Before the change the diagnostic for

    module unknown.submodule {}

was "error: expected module name" which is incorrect and misleading
because both "unknown" and "submodule" are valid module names.

We already have a better error message when a parent module is a
submodule itself and is missing. Make the error for a missing top-level
module more like the one for a submodule.

rdar://problem/64424407

Reviewed By: bruno

Differential Revision: https://reviews.llvm.org/D84458