When substituting a type like T.A.B where A and B are
associated types and the conformance requirement on T.A
is implied by conformance requirements on T, we would
crash.
The problem is that we have no way of representing an
abstract conformance with nested concrete types.
This patch adds a workaround that we can live with until
ConformanceAccessPaths are plumbed through all the way.
Fixes <rdar://problem/30737546> and
<https://bugs.swift.org/browse/SR-3500>.
Possibly fixes <rdar://problem/31334245>.
