[clang] [Clang] Add __builtin_bswapg (PR #162433)

[Aaron Ballman via cfe-commits]

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@@ -12951,6 +12951,9 @@ def err_builtin_invalid_arg_type: Error<
   "%plural{0:|: }3"
   "%plural{[0,3]:type|:types}1 (was %4)">;
 
+def err_bswapg_bitint_not_16bit_aligned : Error<
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AaronBallman wrote:

Not keen on the name here because this implies the issue is with _BitInt that's not aligned to a 16-bit boundary, but that's not what's going on here, right? e.g., `alignas(2) _BitInt(24)` is still not valid to bswap, right?

How about: `err_bswapg_invalid_bit_width`?

Also, does this apply to types other than `_BitInt`? e.g., a vector types, bit-fields, etc

https://github.com/llvm/llvm-project/pull/162433