Remove check for undefined members from Namer.

The Namer had a shallow check for undefined term and type members in
object and anonymous class definitions. This commit fully removes the
check, since RefCheck performs thorough verification of members.

Also add test neg/undefined-members to verify this behavior.

Consequences are:

1. Removal of duplicate code.
2. Allow abstract type members on object and anonymous classes
   (according to spec, see relevant sections below)

Relevant sections of the specification
(as of 32bc5e79d3f293b8a4804959a100f53ec90080ab)

§5.1 Templates (p. 54)
The statement sequence stats contains member definitions that define
new mem- bers or overwrite members in the parent classes. If the
template forms part of an abstract class or trait definition, the
statement part stats may also contain declara- tions of abstract
members. If the template forms part of a concrete class definition,
stats may still contain declarations of abstract type members, but not
of abstract term members.

§5.2 Modifiers (p. 63)
Only abstract classes and traits can have abstract term members.

§5.4 Object Definitions (as per PR scala/scala-dist#127)
Very much like a concrete class definition, an object definition may
still contain declarations of abstract type members, but not of
abstract term members.