[llvm] [LangRef] adjust IR atomics specification following C++20 model tweaks. (PR #77263)

[Fangrui Song via llvm-commits]

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@@ -14,9 +14,16 @@ asynchronous signals.
 The atomic instructions are designed specifically to provide readable IR and
 optimized code generation for the following:
 
-* The C++11 ``<atomic>`` header.  (`C++11 draft available here
-  <http://www.open-std.org/jtc1/sc22/wg21/>`_.) (`C11 draft available here
-  <http://www.open-std.org/jtc1/sc22/wg14/>`_.)
+* The C++ ``<atomic>`` header and C <stdatomic.h> headers. These were
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MaskRay wrote:

`<stdatomic.h>` needs quotes.

https://github.com/llvm/llvm-project/pull/77263