[clang] [clang][docs] Fix DR status for P2280R4 (PR #127166)

[A. Jiang via cfe-commits]

frederick-vs-ja wrote:

@Endilll These checks (modified by #95474) and the newly added [`clang/test/SemaCXX/constant-expression-p2280r4.cpp`](https://github.com/llvm/llvm-project/blob/d3d2ea67585f119c99af66a343f19f237891890e/clang/test/SemaCXX/constant-expression-p2280r4.cpp) are relevant.

https://github.com/llvm/llvm-project/blob/d3d2ea67585f119c99af66a343f19f237891890e/clang/test/SemaCXX/constant-expression-cxx11.cpp#L2431-L2433

https://github.com/llvm/llvm-project/blob/d3d2ea67585f119c99af66a343f19f237891890e/clang/test/SemaCXX/constant-expression-cxx11.cpp#L2437-L2441

Ideally, the C++23 behavior should also be present in C++11~20 modes. Did you want to add FIXME to these checks?

https://github.com/llvm/llvm-project/pull/127166