[llvm] [flang] [clang] [libcxx] [clang-tools-extra] [compiler-rt] [lld] [lldb] [libc] [Clang][C++23] Implement P2448R2: Relaxing some constexpr restrictions (PR #77753)

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> Given the problem in [#77753 (comment)](https://github.com/llvm/llvm-project/pull/77753#issuecomment-1912258038) , @cor3ntin , @AaronBallman WDYT about adding new flags to `CXXRecordDecl`, saying that constructor/destructor is not really `constexpr` before C++23?

Would it make sense to change `DefaultedDestructorIsConstexpr` so it's not a one-bit boolean value but is instead a 2-bit enumeration so we can have `Yes`, `No`, `NotUntilCpp23` as values for it? Then we're not adding related flags but keeping the logic in a single flag?

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