swift234

Revert "Add @effects(readnone) to _swift_isClassOrObjCExistential"

We have not defined whether readnone would potentially allow it to be
speculatively executed which is not desired semantics for
_swift_isClassOrObjCExistential because the metadata might not have been loaded
at the speculative point. Since we don't need the readnone property on this
function after the Builtin.canBeClass change for performance remove it for now.

This reverts commit r21330.

Swift SVN r21422