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

[Joseph Huber via cfe-commits]

jhuber6 wrote:

> > Dropping in after the fact, is there a reason we called this `__builtin_elementwise_ctlz` instead of `__builtin_elementwise_clzg`? The builtin is just `clzg` done on each element so the name is confusing me.
> 
> It matches the llvm intrinsic name, and the second argument is a difference from clz

That is what `clzg` does, unless I'm missing something https://godbolt.org/z/x1fqTxc43.

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