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

[Jakub Kuderski via llvm-commits]

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@@ -1415,6 +1415,18 @@ template <typename ContainerTy> auto make_second_range(ContainerTy &&c) {
       });
 }
 
+/// Return a range that conditionally reverses \p C. The collection is iterated
+/// in reverse if \p Reverse is true (otherwise, it is iterated forwards).
+template <typename ContainerTy>
+[[nodiscard]] auto conditionally_reverse(ContainerTy &&C, bool Reverse) {
+  using ForwardIteratorT = decltype(C.begin());
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kuhar wrote:

Use adl_begin.

I think there's also some utility for this already like `detail::IterOfRange<RangeT>` and `ValueOfRange` -- could we use those?

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