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

[John McCall 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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rjmccall wrote:

This conversation, where I'm requesting that you document which end of the vector is leading/trailing, does not seem resolved. I understand that you find it intuitive that the low indices of the vector are trailing because they map to the least significant bits of the bitmask that you would get with a std::bitcast, but I think that's actually not something that users are going to know. Would you like me to draft the documentation for you?

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