Avoid spurious warnings about forward references in refinements

The warning was triggered by cases like:
class A
type F = A { type T = Int; def f: T }
Which is treated differently from the following which did not produce a warning:
type F = A { type T = Int } { def f: T }
Merge pull request #293 from smarter/fix/multiple-refinements-warning

Avoid spurious warnings about forward references in refinements