d: Fix internal compiler error: in make_import, at d/imports.cc:48 [PR113125]

The cause of the ICE was that TYPE_DECLs were only being generated for
structs with members, not opaque structs.

	PR d/113125

gcc/d/ChangeLog:

	* types.cc (TypeVisitor::visit (TypeStruct *)): Generate TYPE_DECL and
	apply UDAs to opaque struct declarations.

gcc/testsuite/ChangeLog:

	* gdc.dg/imports/pr113125.d: New test.
	* gdc.dg/pr113125.d: New test.