[clang] [clang] Add tests for some of CWG issues resolved in Croydon (2026-03) (PR #189299)

[A. Jiang via cfe-commits]

frederick-vs-ja wrote:

> [P4160R0](https://wiki.isocpp.org/uploads/2026-03_Croydon-StrawPolls_-_p4160r0.html) "Core Language Working Group "ready" Issues for the March, 2026 meeting" is what was voted in, but doesn't seem to be uploaded to publicly accessible WG21 paper system. However, individual Core issues pages linked above contain up-to-date resolutions.

https://wg21.link/p4160r0 already works. I think we can refer to it and update `cxx_dr_status.html` now.

https://github.com/llvm/llvm-project/pull/189299