[swift2objc] Support failable initializers

[liamappelbe]

A failable initializer in Swift is an initializer that can return nil:

public class MyClass {
    public let someInt: Int

    public init?(outerLabel x: Int) {
        if x == 0 {
            return nil
        } else {
            self.someInt = x
        }
    }
}

They show up in the symbolgraph with a ?( token in their second fragment instead of a (:

"declarationFragments": [
    {
        "kind": "keyword",
        "spelling": "init"
    },
    {
        "kind": "text",
        "spelling": "?("
    },

When wrapping these constructors, we have to declare the wrapper initializer as failable, and check if the wrapped initializer returned nil.

@objc public class MyClassWrapper: NSObject {
  var wrappedInstance: MyClass

  @objc init?(outerLabel x: Int) {
    if let instance = MyClass(outerLabel: x) {
      wrappedInstance = instance
    } else {
      return nil
    }
  }
}

[coveralls]

coverage: 90.197% (+0.002%) from 90.195%
when pulling 82f41c5 on failinit
into 8165cbb on main.