swift234

Implement basic support for downcasting from DynamicLookup via postfix '!'.

An expression of DynamicLookup type can be unconditionally downcast to
any class type via the postfix '!'. This will allow one to replace

  var w : NSWindow = (nsarray[0] as NSWindow)!

with

  var w : NSWindow = nsarray[0]!

The current implementation is fairly limited: it only works when the
operand of '!' is an rvalue of type DynamicLookup, and we don't ensure
that the result is a class type. Nonetheless, it cleans up some of
ListMaker nicely.

This is the first client of disjunction constraints, which were added
in r9142.



Swift SVN r9151