swift234

[Constraint solver] Limit the search for type variable bindings based on conformance constraints.

Only consider the list of types that conform to a given protocol as
potential type variable bindings when that protocol has a default type
(i.e., is a literal conversion type). This limits the potential
explosion caused by adding new protocols to which a ton of types
conform, although it's only good for about 3% speedup parsing the
standard library now.

Two minor solver fixes were needed to eliminate our dependence on this
feature:
  - matchTypes() now looks deeper for an underlying fixed type
  - for a tuple type that allows scalar initialization, the type of
  the scalar field is a supertype (for enumeration purposes)



Swift SVN r5837