swift234

Fix: <rdar://problem/23670252> QoI: Misleading error message when assigning a value from [String : AnyObject]

This teaches SubscriptExpr diagnostics to substitute the base type of a
subscript expr through the generic argument types, resolving achetypes in
common situations to concrete types.  We previously complained:

  error: cannot subscript a value of type '[String : AnyObject]' with an index of type 'String'

which is completely wrong, we now produce the correct error:

  error: cannot assign value of type 'AnyObject?' to type 'String?'