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

[Nick Desaulniers 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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nickdesaulniers wrote:

You know this better than everyone else, but `_Generic` statements aren't usable in headers that could be parsed in C++ mode.  So while this macro _could be_ implemented in terms of `_Generic` statements for C mode, you have to do something different for C++, usually duplicating the work.  That's why I think GCC just shipped the type generic builtins.

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