swift234

Use disjunction conversions to handle T -> U? conversion ambiguity.

The constraint solver was eagerly applying the T -> U? conversion
rule, which only succeeds when T is convertible to U. Thus, we would
reject valid code where T has a user-defined conversion to
U?. Instead, introduce a disjunction conversion to try either T -> U?
by converting T to U or via a user-defined conversion. 

Most of this is infrastructure for the introduction of constraint
restrictions, which specify that a particular constraint (say, a
conversion constraint) should only try a single direct path:
scalar-to-tuple, value-to-optional, user-defined, etc. Each term in the
disjunction created for the T -> U? case is restricted to a particular
conversion rule, so that checking it does not create another
disjunction.

Keep track of the constraint restrictions we used within a given
solution, so that we can replay those steps during the application
phase. There is a bunch of inactive code here at the moment, which
will become useful as we start creating disjunctions for other
ambiguous conversions.



Swift SVN r9318