cmd/gc: print expression in 'duplicate case in switch' error

The line number alone does not help when the line is
case '~', '*', '(', ')', '[', ']', '{', '}', '?', ':', ';', ',', '*', '%', '^', '!', '=', '<', '>', '+', '-', '&', '|':

R=ken2
CC=golang-dev
https://golang.org/cl/13431046

src/cmd/gc/swt.c

@@ -415,7 +415,7 @@ mkcaselist(Node *sw, int arg)
 					break;
 				if(!eqtype(c1->node->left->type, c2->node->left->type))
 					continue;
-				yyerrorl(c2->node->lineno, "duplicate case in switch\n\tprevious case at %L", c1->node->lineno);
+				yyerrorl(c2->node->lineno, "duplicate case %T in type switch\n\tprevious case at %L", c2->node->left->type, c1->node->lineno);
 			}
 		}
 		break;
@@ -427,7 +427,7 @@ mkcaselist(Node *sw, int arg)
 			if(exprcmp(c1, c1->link) != 0)
 				continue;
 			setlineno(c1->link->node);
-			yyerror("duplicate case in switch\n\tprevious case at %L", c1->node->lineno);
+			yyerror("duplicate case %N in switch\n\tprevious case at %L", c1->node->left, c1->node->lineno);
 		}
 		break;
 	}