Fixes for Swift troubles
* Remove NS_SWIFT_NAME override for Point, Rect, and Size due to Darwin namespace conflict
* Fix swift_type overrides in objc generator
* Add backwards compatibility Swift typealiases for Point, Rect, Size
* Add disable-swift build option to iOS/macOS builds
* Add import directive to swift source when building with disable-swift
Co-authored-by: Chris Ballinger <cballinger@rightpoint.com>
