cc: broken return is an error, not a warning

R=ken2
CC=golang-dev
https://golang.org/cl/4626081

src/cmd/cc/com.c

@@ -127,7 +127,7 @@ tcomo(Node *n, int f)
 	case ORETURN:
 		if(l == Z) {
 			if(n->type->etype != TVOID)
-				warn(n, "null return of a typed function");
+				diag(n, "null return of a typed function");
 			break;
 		}
 		if(tcom(l))

src/cmd/cc/pgen.c

@@ -112,7 +112,7 @@ codgen(Node *n, Node *nn)
 	warnreach = 1;
 	gen(n);
 	if(canreach && thisfn->link->etype != TVOID)
-		warn(Z, "no return at end of function: %s", n1->sym->name);
+		diag(Z, "no return at end of function: %s", n1->sym->name);
 	noretval(3);
 	gbranch(ORETURN);