Move check for undefined members in objects from Typer to RefCheck

When writing an object (or anonymous classe) with declared but
undefined members, the compiler complains specifically about the
method which is not allowed:
{{{
object Foo {
  def x: Int  // will cause an error message
}
}}}

{{{
error: only classes can have declared but undefined members
  def x: Int
      ^
}}}

This check has been performed by the Typer, unlike other checks for
completeness of classes that are performed in the RefCheck
phase. There are two issues with that:

1. The test in the typer is conceptually wrong and only shallow (it
   e.g. does not check for unimplemented inherited abstract methods)
2. This does not allow compiler plugins to generate such code in a
   post-Typer but pre-RefCheck phase.

This commit moves the check to the RefCheck phase. Error messages
remain the same, except for the additional "object creation
impossible" message emitted from the RefCheck phase. The example code
above will now emit the follwing errors:

{{{
error: object creation impossible, since method x is not defined
object Foo {
       ^
error: only classes can have declared but undefined members
  def x: Int
      ^
}}}