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

[Jakub Kuderski via llvm-commits]

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

I don't think we want to pull in variant in this header;  I'm worried about the  effect on compilation times and some implementations not being optimized very well. Can you implement it without variant?

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