[Clang] Clang erroneously allows `if constexpr` in C23 mode

[Sirraide]

See https://godbolt.org/z/E8PEzzfP3. According to @AaronBallman, this isn’t intentional, so we should probably disallow this.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: None (Sirraide)

See https://godbolt.org/z/E8PEzzfP3. According to @AaronBallman, this isn’t intentional, so we should probably disallow this.

[AaronBallman]

It would be really nice if we could get a fix for this in to Clang 19.x

[Fznamznon]

I don't think this is a regression since clang 18 doesn't support c23 constexpr.

[shafik]

I don't think this is a regression since clang 18 doesn't support c23 constexpr.

I guess I look at it as if constexpr is now erroneously enabled in C whereas previously it was a hard error.