swift234

Introduce a flag to mark classes with Swift-native
refcounting and take advantage of it.

Also, set the Swift1 flag in classes we generate.

Also, initialize a global cache of the non-pointer-isa
mask and use that instead of object_getClass, at least
within the runtime.

Also, centralize the runtime on a _swift_getSuperclass
function and make that use a direct access while we
await word from Greg on the desired ABI requirements.

Swift SVN r21077