[ConstraintSystem] Use fallback type constraint to default pack expansion

Binding of pack expansion types is delayed until solving but use
of `Defaultable` was preventing it from being considered early
because that constraint impacts binding set ranking, switching
to `FallbackType` constraint give us better semantics where pack
expansion type variables are going to be bound as soon as they
have a contextual type.

Resolves: rdar://110819621

lib/Sema/ConstraintSystem.cpp

@@ -1031,7 +1031,7 @@ Type ConstraintSystem::openPackExpansionType(PackExpansionType *expansion,
   // This constraint is important to make sure that pack expansion always
   // has a binding and connect pack expansion var to any type variables
   // that appear in pattern and shape types.
-  addUnsolvedConstraint(Constraint::create(*this, ConstraintKind::Defaultable,
+  addUnsolvedConstraint(Constraint::create(*this, ConstraintKind::FallbackType,
                                            expansionVar, openedPackExpansion,
                                            expansionLoc));