[Bug c++/98940] Implement C++23 language features

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98940
Bug 98940 depends on bug 106650, which changed state.

Bug 106650 Summary: [C++23] P2280 - Using unknown references in constant expressions
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106650

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|ASSIGNED                    |RESOLVED
         Resolution|---                         |FIXED