P3068R6 Allowing Exception Throwing In Constant Evaluation

[StephanTLavavej]

WG21-P3068R6 Allowing Exception Throwing In Constant Evaluation
WG21-P3842R2 Revert constexpr uncaught_exceptions(), current_exception(), nested_exception
LWG-4477 Placement operator delete should be constexpr

This is a Core feature with significant Library impact, including VCRuntime.

Feature-test macro, later increased by #5297:

#define __cpp_lib_constexpr_exceptions 202411L

Note: We are not yet accepting PRs for C++26 features.