[llvm] Update stdckdint.h and make it available in pre-C23 modes. (PR #69649)

[Jessica Clarke via llvm-commits]

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@@ -177,7 +177,7 @@ C23 Feature Support
 - Clang now supports ``N3007 Type inference for object definitions``.
 
 - Clang now supports ``<stdckdint.h>`` which defines several macros for performing
-  checked integer arithmetic.
+  checked integer arithmetic. And it is also exposed in pre-C23 modes.
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jrtc27 wrote:

Don't start a sentence with And

https://github.com/llvm/llvm-project/pull/69649