[clang] [clang] Introduce elementwise ctlz/cttz builtins (PR #131995)

[Matt Arsenault via cfe-commits]

arsenm wrote:

> My literal reading of this is already that it's always undefined behaviour for all targets (as you're requesting),

That's not what I'm requesting, it would be an undefined *value*. It is not instant undefined behavior, you would get UB on use of that value 

https://github.com/llvm/llvm-project/pull/131995