[clang] [clang][bytecode] Implement __builtin_constant_p differently (PR #122099)

[Nikolas Klauser via cfe-commits]

philnik777 wrote:

> > We check which one is the currently active member of a union, and
> 
> This is `__builtin_is_within_lifetime`, isn't it?

I didn't realize that exists. I guess that is the correct builtin, yes.

https://github.com/llvm/llvm-project/pull/122099