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.
Since there still are some holes in this feature, and I haven't had time to
fill them lately: Go back to the 2.2 behavior of treating 'typealias' keyword
in protocols as an associated type, and emit a deprecation warning.
Commented out tests specifically for typealiases in protocols for now, and
random validation tests that crash or not based on whether keyword is interpreted as associatedtype or typealias updated.
