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

[Fangrui Song via llvm-commits]

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@@ -3413,33 +3412,38 @@ For a simpler introduction to the ordering constraints, see the
     stronger) operations on the same address. If an address is written
     ``monotonic``-ally by one thread, and other threads ``monotonic``-ally
     read that address repeatedly, the other threads must eventually see
-    the write. This corresponds to the C++0x/C1x
-    ``memory_order_relaxed``.
+    the write. This corresponds to the C++/C ``memory_order_relaxed``.
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MaskRay wrote:

I was puzzled by `C++/C` formulation as well but was thinking whether jyknight wants to emphasize that C atomics  were based on C++'s :)

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