swift234

[serialization] Break a cyclic dependency involving associated types.

The standard library likes to have default definitions for associated types,
which is good. Often the /choice/ of default type, however, is a type that
(indirectly) conforms to the very protocol containing the associated type.
Rather than try to make sure everything is present all at once, just delay
the deserialization of the default definition until it's actually requested.

This does swell the size of AssociatedTypeDecl by two words. I've filed
<rdar://problem/16266669> to remind myself to try to reduce this.

Part of <rdar://problem/16257259>

Swift SVN r14809