[clang] [libcxx] Reland: [clang] unified CWG2398 and P0522 changes; finishes implementation of P3310 (PR #124137)

[via cfe-commits]

llvmbot wrote:

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Failed to cherry-pick: 28ad8978ee2054298d4198bf10c8cb68730af037

https://github.com/llvm/llvm-project/actions/runs/13209349652

Please manually backport the fix and push it to your github fork.  Once this is done, please create a [pull request](https://github.com/llvm/llvm-project/compare)

https://github.com/llvm/llvm-project/pull/124137