[clang] [clang] Introduce elementwise clz/ctz builtins (PR #131995)

[Eli Friedman via cfe-commits]

efriedma-quic wrote:

> Oh excuse me, there's currently no target-specific behaviour. I missed the HasFallback in

HasFallback only applies to the two-argument form.

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I don't have a strong opinion about the names.

The whole "fallback behavior" thing was written back in the early days of clang, before we even had ubsan.  I doubt we would handle that the same way today... and I don't think we need to carry it forward to new variants.  If the user cares, they can explicitly request `__builtin_elementwise_ctz(x, 32)` or whatever.

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