[clang] Add __builtin_bitreverseg (PR #179126)

[Simon Pilgrim via cfe-commits]

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@@ -119,6 +119,10 @@ Non-comprehensive list of changes in this release
   Usable in constant expressions. Implicit conversion is supported for
   class/struct types with conversion operators.
 
+- A new generaic bit-reverse builtin function ``__builtin_reverseg`` that
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RKSimon wrote:

```suggestion
- A new generic bit-reverse builtin function ``__builtin_reverseg`` that
```

https://github.com/llvm/llvm-project/pull/179126