[clang] [Clang] Support generic bit counting builtins on fixed boolean vectors (PR #154203)

[Joseph Huber via cfe-commits]

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@@ -4213,7 +4213,7 @@ such as ``unsigned __int128`` and C23 ``unsigned _BitInt(N)``.
 
 ``__builtin_clzg`` (respectively ``__builtin_ctzg``) returns the number of
 leading (respectively trailing) 0 bits in the first argument. The first argument
-can be of any unsigned integer type.
+can be of any unsigned integer type or fixed boolean vector.
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jhuber6 wrote:

These boolean vectors basically go back and forth between an integer and a vector of `i1` values in the IR. It would have the same behavior as if the user did `__builtin_bit_cast(uint16_t, v)` or similar. So, leading zeroes would be the ones with 'higher' bit counts, trailing would be lower, same as saying `bitfield >> n & 1` to get the `nth` bit.

https://github.com/llvm/llvm-project/pull/154203