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

[Aaron Ballman via cfe-commits]

https://github.com/AaronBallman approved this pull request.

LGTM aside from a simplification that you can fix up when landing. Thank you for this effort, this is fantastic work!

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