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

[Eli Friedman via cfe-commits]

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@@ -797,6 +797,175 @@ def StdcRotateRight : Builtin {
   let Prototype = "void(...)";
 }
 
+def StdcLeadingZeros: Builtin {
+  let Spellings = ["__builtin_stdc_leading_zeros"];
+  let Attributes = [NoThrow, Const, Constexpr, CustomTypeChecking];
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efriedma-quic wrote:

As long as our implementation satisfies "const", this is fine.

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