Some C APIs take in a const void * that is expected to be a CFURLRef * in certain cases. When a Swift API is generated for these functions, they instead take an UnsafeRawPointer.

Code passing an implicit raw pointer to a struct URL using &var notation can cause issues if the C function assumes the address of a URL points to a NSURL * or CFURLRef. This used to work because an NSURL was the first member of struct URL, but now that memory location contains the _SwiftURL class.

This PR allows us to restore the old behavior in very rare cases on Darwin where it's needed for compatibility by making _BridgedURL an NSObject and forwarding the ObjC messages it receives to the wrapped NSURL.