swift234

Switch over to Generator as the 'for' loop protocol.

Rewrite ForEachStmt SILGen to use the Optional intrinsics with the Generator.next method to iterate through sequences, and kill off the Enumerator path in Sema. Cut over 'EnumeratorType.Element' requirements to instead require 'GeneratorType.Element' in the stdlib.

There are a couple of bugs remaining that need follow-up work. There appears to be a bug in nested enum layout (e.g. T??) that's causing test/Interpreter/enum to break; I'll investigate and fix. There's also a lingering type-checker bug with inferred associated types that causes them to fail requirement checks <rdar://problem/15172101>, which I think Doug needs to look into.

Swift SVN r9017