[llvm] [ADT] Series of improvements to SmallSet (PR #104611)

[Sergei Barannikov via llvm-commits]

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@@ -155,6 +150,22 @@ class SmallSet {
   using const_iterator = SmallSetIterator<T, N, C>;
 
   SmallSet() = default;
+  SmallSet(const SmallSet &) = default;
+  SmallSet(SmallSet &&) = default;
+
+  template <typename IterT> explicit SmallSet(IterT Begin, IterT End) {
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s-barannikov wrote:

```suggestion
  template <typename IterT> SmallSet(IterT Begin, IterT End) {
```

https://github.com/llvm/llvm-project/pull/104611