[swift2objc] Add support for private setters on variables

[nikeokoronkwo]

From https://docs.swift.org/swift-book/documentation/the-swift-programming-language/accesscontrol#Getters-and-Setters:

You can give a setter a lower access level than its corresponding getter, to restrict the read-write scope of that variable, property, or subscript. You assign a lower access level by writing fileprivate(set), private(set), internal(set), or package(set) before the var or subscript introducer.

When exporting to Objective-C, the properties are marked as readonly. See the example below

class MyClass {
  public var myPublicProperty: String
  public private(set) var myPrivateSetterProperty: String
}

// implementation in Objective-C
@interface MyClass : NSObject
@property (nonatomic) NSString myPublicProperty;
@property (nonatomic, readonly) NSString myPublicProperty;
@end