swift234

Fix rdar://21783216 QoI: Cannot convert {TYPE} to {SAME TYPE}
which was reported here: https://twitter.com/jadengeller/status/619989059046240256

The underlying problem here is that the user was defining an associated
type named "Type", and then trying to refer to it with stuff.Type.  The
problem is that stuff.Type is a reserved way to refer to the metatype.

Solve this sort of confusion by banning type members named Type (and
Protocol, while we're here) since forming a reference to them won't
work.  This produces a note that indicates that a backtick'd version
of the identifier will work, since "stuff.`Type`" will correctly form
the reference to it.

This only bans type members named Type or Protocol, but we could consider
banning all ValueDecls from being named Type or Protocol.  Module
qualification isn't widely used though, and metatypes of modules don't
really make sense at the moment.