[Bug c++/88323] implement C++20 language features.

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88323
Bug 88323 depends on bug 91360, which changed state.

Bug 91360 Summary: Implement P1143R2: constinit
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91360

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