[clang] [clang] Implement constexpr support for __builtin_{clzg, ctzg} (PR #86577)

[via cfe-commits]

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@@ -188,6 +188,11 @@ Non-comprehensive list of changes in this release
 
 - Lambda expressions are now accepted in C++03 mode as an extension.
 
+- Added ``__builtin_clzg`` and ``__builtin_ctzg`` as type-generic alternatives
+  to ``__builtin_clz{,s,l,ll}`` and ``__builtin_ctz{,s,l,ll}`` respectively,
+  with support for any unsigned integer type. Like the previous builtins, these
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overmighty wrote:

I had forgotten to add a release note for the addition of these builtins in #83431.

https://github.com/llvm/llvm-project/pull/86577