c++/modules: Fix ICE with multiple module declarations and -M [PR123738]

c++/modules: Fix ICE with multiple module declarations and -M [PR123738]

When only doing preprocessing, we do not call declare_module, and so we
do not check that a module has been declared multiple times.  This means
we end up with multiple "primary modules" being passed to the dependency
producing logic, which asserts.  Fixed by checking if we've already seen
a module declaration and complaining.

	PR c++/123738

gcc/cp/ChangeLog:

	* module.cc (preprocess_module): Complain for any non-imported
	module declaration in a module purview.

gcc/testsuite/ChangeLog:

	* g++.dg/modules/dep-5.C: New test.

Signed-off-by: Nathaniel Shead <nathanieloshead@gmail.com>
Reviewed-by: Jason Merrill <jason@redhat.com>