c++: fix PR104025 change

Somehow I pushed my earlier patch without it actually fixing the test; we
need input_location to be for the last consumed token, not the next one.

gcc/cp/ChangeLog:

	* parser.cc (saved_token_sentinel::rollback): Use
	cp_lexer_previous_token.

gcc/cp/parser.cc

@@ -1308,7 +1308,8 @@ struct saved_token_sentinel
   void rollback ()
   {
     cp_lexer_rollback_tokens (lexer);
-    cp_lexer_set_source_position (lexer);
+    cp_lexer_set_source_position_from_token
+      (cp_lexer_previous_token (lexer));
     mode = STS_DONOTHING;
   }
   ~saved_token_sentinel ()