c++: XFAIL testcase for PR98019

Apparently it isn't actually fixed on trunk yet, was just passing because of
some WIP in my tree.  So XFAIL for now.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/concepts-nodiscard1.C: XFAIL.

gcc/testsuite/g++.dg/cpp2a/concepts-nodiscard1.C

@@ -1,5 +1,6 @@
 // PR c++/98019
 // { dg-do compile { target c++20 } }
+// { dg-excess-errors *-*-* }
 
 template <class T, class U> concept same_as = __is_same_as (T, U);