Before we create a new initializer, check whether it would collide
with existing initializers. If it's better than the existing
initializer, mark the existing one as unavailable; if it's not better,
don't build it.
In support of this, we tweak a few things w.r.t. unavailble
declarations:
- An unavailable declaration is shadowed by an available one,
- Don't emit SIL unavailable, imported initializers
This fixes the last problem with <rdar://problem/16509024>.
Swift SVN r16611
