SI-8217 allow abstract type members in objects

Previously, abstract type members were allowed in objects only when inherited,
but not when declared directly. This inconsistency was not intended. In dotty,
abstract type members are allowed in values and represent existentials; so upon
discussion, it was decided to fix things to conform to dotty and allow such type
members. Adriaan also asked to keep rejecting abstract type members in methods
even though they would conceivably make sense.

Discussions happened on #3407, scala/scala-dist#127.
This code is improved from #3442, keeps closer to the current logic, and passes tests.

Existing tests that have been converted to `pos` tests show that
this works, and a new test has been added to show that local
aliases (ie term-owned) without a RHS are still rejected.

I've just added the new test to @Blaisorblade's PR, #3978.

Review by @adriaanm

I tend to think this one is okay for 2.11.x, as it only loosens
a restriction. That said, it would be no great loss to wait until
2.12.