swift234

Use disjunctions to disambiguiate existential vs. user-defined conversions.

As with the T -> U? conversion ambiguity addressed in r9316, we need
to explore both the to-existential conversion and the user-defined
conversion separately. Do so with a disjunction constraint.

This allows us to eliminate the heinous hack I added for the
user-defined _Nil -> DynamicLookup conversion.



Swift SVN r9333