[clang] [C23] Implement N3018: The constexpr specifier for object definitions (PR #73099)

[Mariya Podchishchaeva via cfe-commits]

Fznamznon wrote:

> Please let me know your thoughts.

@AaronBallman , implemented in https://github.com/llvm/llvm-project/pull/73099/commits/dff61e0f49d0ba0d868f4e9668f75718b031e1e3 .
I was mostly referring to the original page https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3018.htm when implementing the feature, so these NaN cases slipped.

https://github.com/llvm/llvm-project/pull/73099