[clang] [C23] Add __builtin_c23_va_start (PR #131166)

[Aaron Ballman via cfe-commits]

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@@ -833,6 +833,12 @@ def BuiltinVaStart : Builtin {
   let Prototype = "void(__builtin_va_list_ref, ...)";
 }
 
+def BuiltinC32VaStart : LangBuiltin<"C23_LANG"> {
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AaronBallman wrote:

This matches GCC's behavior: https://godbolt.org/z/K8Tdqa9v4

https://github.com/llvm/llvm-project/pull/131166