[clang] [clang] Implement __builtin_{clzg,ctzg} (PR #83431)

[Nick Desaulniers via cfe-commits]

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@@ -660,15 +660,23 @@ def Clz : Builtin, BitShort_Int_Long_LongLongTemplate {
   let Prototype = "int(unsigned T)";
 }
 
-// FIXME: Add int clzimax(uintmax_t)
+def Clzg : Builtin {
+  let Spellings = ["__builtin_clzg"];
+  let Attributes = [NoThrow, Const, CustomTypeChecking];
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nickdesaulniers wrote:

Should `Constexpr` be in this list of attributes?

https://godbolt.org/z/Efd3EW5xE

https://github.com/llvm/llvm-project/pull/83431