swift234

remove the prohibition of weak pointers being implicitly unwrapped, and update some testcases I
missed on previous commits.  This fixes:
<rdar://problem/16792095> QoI: misleading error message when trying to make a weak property be implicitly-unwrapped optional

and enables a useful pattern, where implicitly unwrapped weak pointers don't require !'ing on every use.



Swift SVN r17940