Move check for undefined members in objects from Namer to RefCheck
When writing an object (or an anonymous class) 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
    }

will emit

    test.scala:16: error: only classes can have declared but undefined members
      def x: Int
          ^

This check has been performed by the Namer, 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 Namer 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.
3. The testing code exists twice. So this commit removes some
   duplicate code.

This commit moves the check to the RefCheck phase. The example code
above will now emit the following errors:

    test.scala:15: error: object creation impossible, since method x is not defined
    object Foo {
           ^

The new test undefined-members verifies that behavior stays the same.