[llvm] [ADT] Backport std::to_underlying from C++23 (PR #70681)

[Vlad Serebrennikov via llvm-commits]

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@@ -1535,6 +1535,13 @@ constexpr decltype(auto) makeVisitor(CallableTs &&...Callables) {
   return detail::Visitor<CallableTs...>(std::forward<CallableTs>(Callables)...);
 }
 
+/// Backport of C++23 std::to_underlying.
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Endilll wrote:

Done!

https://github.com/llvm/llvm-project/pull/70681