swift234

Handle uses of mutating methods of protocol extensions on existentials.

To properly deal with mutating methods on protocol extensions accessed through an existential (such as a property or subscript setter), open the existential as an lvalue so that its value can be mutated. 

As part of this, make sure we form any OpenExistentialExprs at the top level of the expression if we didn't manage to build them sooner. This is a fairly ad hoc approach that I am not happy with (these could probably be placed better with a post-pass), but the cost of opening too early is fairly minimal.

Swift SVN r27068