Add support for Clang 22 __builtin_ct_select

[bhess]

LLVM/Clang 22 will introduce a new function, __builtin_ct_select, which guarantees constant-time behavior by preventing compiler optimizations that could introduce branching-based timing attacks.

This proposal suggests adding a common interface that uses this built-in when compiled with Clang 22. Using this interface would improve robustness against compiler-induced regressions in constant-time code.

For more background, see: https://blog.trailofbits.com/2025/12/02/introducing-constant-time-support-for-llvm-to-protect-cryptographic-code/

Note: This is future work, as LLVM 22 has not yet been released.

[mkannwischer]

Do you happen to know if that feature surely will make it into LLVM22? The PR linked from the blog post has not been merged.
There is a rc1 of LLVM22 now, but I can't find any mention of it anywhere.