add return self and init

Author: mkandil

stdlib/public/Cxx/CxxDictionary.swift

@@ -25,6 +25,8 @@ public protocol CxxDictionary<Key, Value> {
   associatedtype Size: BinaryInteger
   associatedtype InsertionResult
 
+  init()
+
   /// Do not implement this function manually in Swift.
   func __findUnsafe(_ key: Key) -> RawIterator
 
@@ -76,10 +78,10 @@ extension CxxDictionary {
     }
   }
 
-  func filter(
+  public func filter(
     _ isIncluded: (_ key: Key, _ value: Value) throws -> Bool
-) rethrows -> [Key: Value] {
-    var filteredDictionary: [Key: Value] = [:]
+) rethrows -> Self {
+    var filteredDictionary: Self.init()
     var iterator = __beginUnsafe() 
     let endIterator = __endUnsafe()