c++: Add testcase for CWG2575 [PR120778]

c++: Add testcase for CWG2575 [PR120778]

From the paper it isn't clear what caused the decision changes, not to drop
the "the token defined is generated as a result of this replacement process or"
part and make [cpp.cond]/10 violations IFNDR rather than ill-formed (the
latter maybe so that the extension to handle e.g. !A(A) below etc. can be
accepted).

Anyway, because that case hasn't been dropped and we pedwarn on it already,
and diagnose everything else the way it should, the following patch just
adds testcase for it.

2025-08-07  Jakub Jelinek  <jakub@redhat.com>

	PR preprocessor/120778
	* g++.dg/DRs/dr2575.C: New test.