[clang] Update C++ DR status page (#121642)
This patch brings our C++ DR status page up to date with WG21 updates.
[CWG1223](https://cplusplus.github.io/CWG/issues/1223.html) "Syntactic
disambiguation and _trailing-return-types_" is resolved by
[P2915R0](https://wg21.link/p2915r0) "Proposed resolution for CWG1223".
Both the test and the paper were written by @cor3ntin, so I presume no
updates are needed.
[CWG2819](https://cplusplus.github.io/CWG/issues/2819.html) "Cast from
null pointer value in a constant expression" was revisited and marked as
not a DR, so I updated the test to ensure that the example is not
accepted in C++23 and earlier modes. CC @offsetof.
Tentantive resolutions to the following issues were simply promoted to
actual resolutions, so tests don't require any meaningful changes:
- [CWG2913](https://cplusplus.github.io/CWG/issues/2913.html) "Grammar
for deduction-guide has requires-clause in the wrong position"
(@zyn0217)
- [CWG2915](https://cplusplus.github.io/CWG/issues/2915.html) "Explicit
object parameters of type void" (@MitalAshok)
- [CWG2922](https://cplusplus.github.io/CWG/issues/2922.html) "constexpr
placement-new is too permissive" (@cor3ntin)
As a drive-by fix, I updated the `make_cxx_dr_status` script to
accommodate for `C++23 onwards` and `C++26 onwards` statuses, which are
useful for Core issues that are not DRs.
