Improve type inferencing for Spock expect block

Consider the following:
```groovy
class Foo {
  String bar
}
final class FooTest exteds spock.lang.Specification {
  void 'test case'() {
   given:
    Foo foo = new Foo(bar: 42)
   expect:
    foo.bar == 42
  }
}
```

If the Spock global transform is enabled (soon it will always be enabled: #814), "bar" is showing as unknown (underlined).