cmd/gc: rephrase the invalid indexing operation error message

Old:
prog.go:9: invalid operation: this[i] (index of type int)
New:
prog.go:9: invalid operation: this[i] (type int does not support indexing)

LGTM=r
R=golang-codereviews, r
CC=golang-codereviews
https://golang.org/cl/52540043

src/cmd/gc/typecheck.c

@@ -866,7 +866,7 @@ reswitch:
 			goto error;
 		switch(t->etype) {
 		default:
-			yyerror("invalid operation: %N (index of type %T)", n, t);
+			yyerror("invalid operation: %N (type %T does not support indexing)", n, t);
 			goto error;