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

[via cfe-commits]

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@@ -1,4 +1,5 @@
 // NOTE: Assertions have been autogenerated by utils/update_cc_test_checks.py UTC_ARGS: --version 3
+// RUN: %clang_cc1 -triple=x86_64 -verify -ffreestanding -std=c2x %s
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ZijunZhaoCCK wrote:

Nope, I just want to test c20 and c23. But yes I think I should add `std=c11/c17/...` like below 

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