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

[Aaron Ballman 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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AaronBallman wrote:

Ah! There is no c20 release. There is C89/C90, C99, C11/C1x, C17/C18/C1y, and C23/C2x

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