swift234

IRGen: Lower `super_method` SIL instruction for native classes

This is the first part of making class method dispatch resilient.

If we have the following class hierarchy:

    // Module A
    class Parent {
      func foo() {}
    }

    class Child : Parent {}

    // Module B
    class Grandchild : Child {
      override func foo() {
        super.foo()
      }
    }

dispatch to `Parent.foo` will be static via a `function_ref`, so if
someone adds a `foo()` to `Child` later on, `Grandchild` won't know
about it without recompiling.

Stage in the IRGen portion of dynamic dispatch when calling methods on
`super`:

- Don't assert in IRGen if we see a native super_method instruction.
- Perform virtual lookup on superclass's metadata. If we see a
  `super_method` instruction for a native class:
  - Get the address of the superclass's metadata at offset 1
  - Load the superclass's metadata
  - Perform virtual lookup on this metadata instead

TODO: SILGen super_method instructions for native classes.
TODO: Devirtualize back down to static dispatch with a reslience lookup
mechanism.

rdar://problem/22749732