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

[Alexander Richardson via cfe-commits]

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@@ -755,6 +755,12 @@ def BSwap : Builtin, Template<["unsigned short", "uint32_t", "uint64_t"],
   let Prototype = "T(T)";
 }
 
+def BSwapg : Builtin {
+  let Spellings = ["__builtin_bswapg"];
+  let Attributes = [NoThrow, Const, Constexpr, CustomTypeChecking];
+  let Prototype = "int(...)";
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arichardson wrote:

```suggestion
  let Prototype = "T(T)";
```
Should this match the other generic ones?

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