[llvm] [ADT] Add `llvm::conditionally_reverse()` iterator (PR #171040)

[Benjamin Maxwell via llvm-commits]

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@@ -40,6 +40,7 @@
 #include <tuple>
 #include <type_traits>
 #include <utility>
+#include <variant>
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MacDue wrote:

I've implemented it with `zip_equal` instead (which was my original approach anyway). This is slightly worse, since it will iterate in both directions, but it mostly seems fine: https://godbolt.org/z/MTPKn65bK

https://github.com/llvm/llvm-project/pull/171040