[clang] [clang] Define ATOMIC_FLAG_INIT correctly for C++. (PR #97534)

[Aaron Ballman via cfe-commits]

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@@ -750,6 +750,8 @@ Bug Fixes in This Version
 
 - Fixed `static_cast` to array of unknown bound. Fixes (#GH62863).
 
+- Fixed the definition of ATOMIC_FLAG_INIT in stdatomic.h so it can be used in C++.
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AaronBallman wrote:

```suggestion
- Fixed the definition of ``ATOMIC_FLAG_INIT`` in ``<stdatomic.h>`` so it can be used in C++.
```

https://github.com/llvm/llvm-project/pull/97534