Restore ImplicitlyUnwrappedOptional extension conforming to _ObjectiveCBridgeable.

Partial revert of 26f6a75.

Removing this breaks bridging these values to Objective C. Once IUOs
are removed from the type system and ImplicitlyUnwrappedOptional<T> is
removed from the library, we'll be strictly using Optional<T> which
has this conformance as well.

Author: rudkx

stdlib/public/core/ImplicitlyUnwrappedOptional.swift

@@ -49,3 +49,48 @@ public enum ImplicitlyUnwrappedOptional<Wrapped> : ExpressibleByNilLiteral {
     self = .none
   }
 }
+
+#if _runtime(_ObjC)
+extension ImplicitlyUnwrappedOptional : _ObjectiveCBridgeable {
+  @_inlineable // FIXME(sil-serialize-all)
+  public func _bridgeToObjectiveC() -> AnyObject {
+    switch self {
+    case .none:
+      _preconditionFailure("Attempt to bridge an implicitly unwrapped optional containing nil")
+
+    case .some(let x):
+      return Swift._bridgeAnythingToObjectiveC(x)
+    }
+  }
+
+  @_inlineable // FIXME(sil-serialize-all)
+  public static func _forceBridgeFromObjectiveC(
+    _ x: AnyObject,
+    result: inout ImplicitlyUnwrappedOptional<Wrapped>?
+  ) {
+    result = Swift._forceBridgeFromObjectiveC(x, Wrapped.self)
+  }
+
+  @_inlineable // FIXME(sil-serialize-all)
+  public static func _conditionallyBridgeFromObjectiveC(
+    _ x: AnyObject,
+    result: inout ImplicitlyUnwrappedOptional<Wrapped>?
+  ) -> Bool {
+    let bridged: Wrapped? =
+      Swift._conditionallyBridgeFromObjectiveC(x, Wrapped.self)
+    if let value = bridged {
+      result = value
+    }
+
+    return false
+  }
+
+  @_inlineable // FIXME(sil-serialize-all)
+  public static func _unconditionallyBridgeFromObjectiveC(_ source: AnyObject?)
+      -> Wrapped! {
+    var result: ImplicitlyUnwrappedOptional<Wrapped>?
+    _forceBridgeFromObjectiveC(source!, result: &result)
+    return result!
+  }
+}
+#endif