[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 91368, which changed state.

Bug 91368 Summary: Implement P1301R4: [[nodiscard("with reason")]]
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91368

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