[swift2objc] Support `throws` on methods returning primitive types

[liamappelbe]

Hitting this compile error when trying to support the throws annotation:

pkgs/swift2objc/test/integration/throws_output.swift:16:21: error: throwing method cannot be marked @objc because it returns a value of type 'Int'; return 'Void' or a type that bridges to an Objective-C class
  @objc public func intMethod(y: Int) throws -> Int {

Looks like functions and methods that return primitive types can't be marked both @objc and throws. This is similar to #1743. The fix in both cases is to return a boxed primitive.