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

[Timm Baeder via cfe-commits]

tbaederr wrote:

> We check which one is the currently active member of a union, and

This is `__builtin_is_within_lifetime`, isn't it?

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