[stdlib] Optimize Set.isStrictSuperset(of:)

- Use a temporary bitset to speed up the `Sequence` variant by roughly a factor of 4.
- Fix a logic error causing the `a == b` case for the set variant to be O(n) instead of O(1).

Author: lorentey

stdlib/public/core/NativeSet.swift

@@ -636,4 +636,25 @@ extension _NativeSet {
       return false
     }
   }
+
+  @_alwaysEmitIntoClient
+  internal func isStrictSuperset<S: Sequence>(of possibleSubset: S) -> Bool
+  where S.Element == Element {
+    _UnsafeBitset.withTemporaryBitset(capacity: self.bucketCount) { seen in
+      // Mark elements in self that we've seen in `possibleStrictSubset`.
+      var seenCount = 0
+      for element in possibleSubset {
+        let (bucket, found) = find(element)
+        guard found else { return false }
+        let inserted = seen.uncheckedInsert(bucket.offset)
+        if inserted {
+          seenCount += 1
+          if seenCount == self.count {
+            return false
+          }
+        }
+      }
+      return true
+    }
+  }
 }

stdlib/public/core/Set.swift

@@ -796,8 +796,11 @@ extension Set: SetAlgebra {
   @inlinable
   public func isStrictSuperset<S: Sequence>(of possibleStrictSubset: S) -> Bool
   where S.Element == Element {
-    let other = Set(possibleStrictSubset)
-    return other.isStrictSubset(of: self)
+    if isEmpty { return false }
+    if let s = possibleStrictSubset as? Set<Element> {
+      return isStrictSuperset(of: s)
+    }
+    return _variant.convertedToNative.isStrictSuperset(of: possibleStrictSubset)
   }
 
   /// Returns a Boolean value that indicates whether the set has no members in
@@ -1191,7 +1194,7 @@ extension Set {
   ///   `other`; otherwise, `false`.
   @inlinable
   public func isStrictSuperset(of other: Set<Element>) -> Bool {
-    return self.isSuperset(of: other) && self != other
+    return self.count > other.count && other.isSubset(of: self)
   }
 
   /// Returns a Boolean value that indicates whether the set is a strict subset