[clang] [clang]Implement the c23 stdc bit builtins (PR #185978)

[Aaron Ballman via cfe-commits]

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@@ -0,0 +1,19 @@
+#ifndef LLVM_CLANG_TEST_STDBIT_H
+#define LLVM_CLANG_TEST_STDBIT_H
+
+#define stdc_leading_zeros(x) (__builtin_stdc_leading_zeros((x)))
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AaronBallman wrote:

Ope, maybe I spoke too soon. C23 7.18.1p5:

> It is unspecified whether any generic function declared in <stdbit.h> is a macro or an identifier
declared with external linkage. If a macro definition is suppressed to access an actual function,
or a program defines an external identifier with the name of a generic function, the behavior is
unspecified.

https://github.com/llvm/llvm-project/pull/185978