d: Fix internal compiler error: in visit, at d/decl.cc:838 [PR119799]

This was caused by a check in the D front-end disallowing static
VAR_DECLs with a size `0'.

While empty structs in D are give the size `1', the same symbol coming
from ImportC modules do infact have no size, so allow C variables to
pass the check as well as array objects.

	PR d/119799

gcc/d/ChangeLog:

	* decl.cc (DeclVisitor::visit (VarDeclaration *)): Check front-end
	type size before building the VAR_DECL.  Allow C symbols to have a
	size of `0'.

gcc/testsuite/ChangeLog:

	* gdc.dg/import-c/pr119799.d: New test.
	* gdc.dg/import-c/pr119799c.c: New test.

gcc/d/decl.cc

@@ -791,6 +791,12 @@ public:
       }
     else if (d->isDataseg ())
       {
+	/* When the front-end type size is invalid, an error has already been
+	   given for the declaration or type.  */
+	dinteger_t size = dmd::size (d->type, d->loc);
+	if (size == SIZE_INVALID)
+	  return;
+
 	tree decl = get_symbol_decl (d);
 
 	/* Only need to build the VAR_DECL for extern declarations.  */
@@ -804,9 +810,7 @@ public:
 	  return;
 
 	/* How big a symbol can be should depend on back-end.  */
-	tree size = build_integer_cst (dmd::size (d->type, d->loc),
-				       build_ctype (Type::tsize_t));
-	if (!valid_constant_size_p (size))
+	if (!valid_constant_size_p (build_integer_cst (size, size_type_node)))
 	  {
 	    error_at (make_location_t (d->loc), "size is too large");
 	    return;
@@ -835,8 +839,9 @@ public:
 	  }
 
 	/* Frontend should have already caught this.  */
-	gcc_assert (!integer_zerop (size)
-		    || d->type->toBasetype ()->isTypeSArray ());
+	gcc_assert ((size != 0 && size != SIZE_INVALID)
+		    || d->type->toBasetype ()->isTypeSArray ()
+		    || d->isCsymbol ());
 
 	d_finish_decl (decl);
 

gcc/testsuite/gdc.dg/import-c/pr119799.d

@@ -0,0 +1,2 @@
+// { dg-do compile }
+import pr119799c;

gcc/testsuite/gdc.dg/import-c/pr119799c.c

@@ -0,0 +1 @@
+static struct {} s119799;