gc: disallow ... in type conversions

Fixes #1866.

R=bradfitz, rsc
CC=golang-dev
https://golang.org/cl/4548073

src/cmd/gc/typecheck.c

@@ -794,7 +794,7 @@ reswitch:
 		defaultlit(&n->left, T);
 		l = n->left;
 		if(l->op == OTYPE) {
-			if(n->isddd)
+			if(n->isddd || l->type->bound == -100)
 				yyerror("invalid use of ... in type conversion", l);
 			// pick off before type-checking arguments
 			ok |= Erv;

test/ddd1.go

@@ -43,4 +43,5 @@ func bad(args ...int) {
 	var x int
 	_ = unsafe.Pointer(&x...)	// ERROR "[.][.][.]"
 	_ = unsafe.Sizeof(x...)	// ERROR "[.][.][.]"
+	_ = [...]byte("foo") // ERROR "[.][.][.]"
 }