swift234

Disable typealiases in protocols and add frontend flag to re-enable.

Goes back to Swift 2.2 behavior of treating the 'typealias' keyword inside a protocol as a deprecated form of an associatedtype. To get the newer (but still partly buggy) behavior of treating it as an actual typealias, add "-Xfrontend -enable-protocol-typealiases" to the compile invocation. 'decl/typealias/typealias.swift' now uses this flag to continue testing the enabled behavior.