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.

gcc/d/types.cc

@@ -1230,6 +1230,11 @@ public:
 	apply_user_attributes (t->sym, t->ctype);
 	finish_aggregate_type (structsize, alignsize, t->ctype);
       }
+    else
+      {
+	build_type_decl (t->ctype, t->sym);
+	apply_user_attributes (t->sym, t->ctype);
+      }
 
     /* For structs with a user defined postblit, copy constructor, or a
        destructor, also set TREE_ADDRESSABLE on the type and all variants.

gcc/testsuite/gdc.dg/imports/pr113125.d

@@ -0,0 +1,2 @@
+module imports.pr113125;
+struct S113125;

gcc/testsuite/gdc.dg/pr113125.d

@@ -0,0 +1,4 @@
+// { dg-do compile }
+// { dg-options "-I $srcdir/gdc.dg" }
+module pr113125;
+import imports.pr113125: S113125;