[ADT] Add a 'find_as' operation to DenseSet.

This operation is analogous to its counterpart in DenseMap: It allows lookup
via cheap-to-construct keys (provided that getHashValue and isEqual are
implemented for the cheap key-type in the DenseMapInfo specialization).

Thanks to Chandler for the review.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@220168 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/ADT/DenseSet.h

@@ -110,6 +110,21 @@ public:
   const_iterator end() const { return ConstIterator(TheMap.end()); }
 
   iterator find(const ValueT &V) { return Iterator(TheMap.find(V)); }
+
+  /// Alternative version of find() which allows a different, and possibly less
+  /// expensive, key type.
+  /// The DenseMapInfo is responsible for supplying methods
+  /// getHashValue(LookupKeyT) and isEqual(LookupKeyT, KeyT) for each key type
+  /// used.
+  template <class LookupKeyT>
+  iterator find_as(const LookupKeyT &Val) {
+    return Iterator(TheMap.find_as(Val));
+  }
+  template <class LookupKeyT>
+  const_iterator find_as(const LookupKeyT &Val) const {
+    return ConstIterator(TheMap.find_as(Val));
+  }
+
   void erase(Iterator I) { return TheMap.erase(I.I); }
   void erase(ConstIterator CI) { return TheMap.erase(CI.I); }