2 crashes with generic extension convenience init

Two crashes in Xcode 6.1 Beta 3.

```
  ✘  238 swift lowering typeconverter getconstantinfo                                 (8dfde75e55)
  ✘  239 swiftdeclconverter importconstructor                                         (ad0eb11ca7)
```

Tests only work correctly with pull request #20 applied.