[ConstraintSystem] Make `typeCheckParameterDefault` a friend
[ConstraintSystem] Make `typeCheckParameterDefault` a friend

`typeCheckParameterDefault` needs access to `applySolution`
in order to apply a solution found in `inference from defaults` mode.

benlangmuir/swift,ahoppen/swift,apple/swift,rudkx/swift,roambotics/swift,ahoppen/swift,benlangmuir/swift,glessard/swift,rudkx/swift,apple/swift,benlangmuir/swift,benlangmuir/swift,gregomni/swift,JGiola/swift,rudkx/swift,rudkx/swift,roambotics/swift,atrick/swift,glessard/swift,JGiola/swift,ahoppen/swift,gregomni/swift,roambotics/swift,glessard/swift,apple/swift,benlangmuir/swift,roambotics/swift,gregomni/swift,glessard/swift,roambotics/swift,apple/swift,gregomni/swift,gregomni/swift,ahoppen/swift,ahoppen/swift,JGiola/swift,glessard/swift,atrick/swift,apple/swift,JGiola/swift,roambotics/swift,atrick/swift,glessard/swift,JGiola/swift,gregomni/swift,benlangmuir/swift,atrick/swift,apple/swift,rudkx/swift,atrick/swift,rudkx/swift,ahoppen/swift,atrick/swift,JGiola/swift