[clang] Add clang atomic control options and attribute (PR #114841)

[Hubert Tong via cfe-commits]

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@@ -5442,6 +5442,155 @@ third argument, can only occur at file scope.
     a = b[i] * c[i] + e;
   }
 
+Extensions for controlling atomic code generation
+=================================================
+
+The ``[[clang::atomic]]`` statement attribute enables users to control how
+atomic operations are lowered in LLVM IR by conveying additional metadata to
+the backend. The primary goal is to allow users to specify certain options,
+like whether atomic operations may be performed on specific types of memory or
+whether to ignore denormal mode correctness in floating-point operations,
+without affecting the correctness of code that does not rely on these behaviors.
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hubert-reinterpretcast wrote:

s/behaviors/properties/;

https://github.com/llvm/llvm-project/pull/114841