[llvm] [clang-tools-extra] [clang] [Clang][C++23] Implement P2448R2: Relaxing some constexpr restrictions (PR #77753)

[Aaron Ballman via cfe-commits]

================
@@ -2765,10 +2765,14 @@ def err_constexpr_tag : Error<
   "cannot be marked %sub{select_constexpr_spec_kind}1">;
 def err_constexpr_dtor : Error<
   "destructor cannot be declared %sub{select_constexpr_spec_kind}0">;
-def err_constexpr_dtor_subobject : Error<
-  "destructor cannot be declared %sub{select_constexpr_spec_kind}0 because "
+def ext_constexpr_dtor_subobject : ExtWarn<
+  "destructor cannot be declared %sub{select_constexpr_spec_kind}0 before C++23 because "
   "%select{data member %2|base class %3}1 does not have a "
-  "constexpr destructor">;
+  "constexpr destructor">, InGroup<CXX23>, DefaultError;
+def warn_cxx23_compat_constexpr_dtor_subobject : ExtWarn<
----------------
AaronBallman wrote:

```suggestion
def warn_cxx23_compat_constexpr_dtor_subobject : Warning<
```
this part isn't an extension, just a regular warning.

https://github.com/llvm/llvm-project/pull/77753