swift234

Tighten the constraint on the base of a '.' expression for protocol members.

The "conversion" constraint was far too loose, because we don't want
to permit arbitrary conversions on the left-hand side of the
'.'. Conformance constraints aren't correct either, because an
existential that does not conform to itself can still be used on the
left-hand side of a '.', so we introduce a new kind of constraint for
this.


Swift SVN r9630