[ConstraintSystem] Use fallback type constraint to default pack expansion #66708
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Generalizes
DefaultClosureType
intoFallbackType
constraint.Binding of pack expansion types is delayed until solving but use
of
Defaultable
was preventing it from being considered earlybecause that constraint impacts binding set ranking, switching
to
FallbackType
constraint give us better semantics where packexpansion type variables are going to be bound as soon as they
have a contextual type.
Resolves: rdar://110819621