c++: fix cpp0x/constexpr-ex1.C in C++23
Since r14-6505 I see:
FAIL: g++.dg/cpp0x/constexpr-ex1.C -std=c++23 at line 91 (test for errors, line 89)
FAIL: g++.dg/cpp0x/constexpr-ex1.C -std=c++23 (test for excess errors)
FAIL: g++.dg/cpp0x/constexpr-ex1.C -std=c++26 at line 91 (test for errors, line 89)
FAIL: g++.dg/cpp0x/constexpr-ex1.C -std=c++26 (test for excess errors)
and it wasn't fixed by r14-6511. So I'm fixing it with the below.
gcc/testsuite/ChangeLog:
* g++.dg/cpp0x/constexpr-ex1.C: Adjust expected diagnostic line.