[ConstraintSystem] NFC: Make `SemaTest` a friend of `ConstraintSystem`
[ConstraintSystem] NFC: Make `SemaTest` a friend of `ConstraintSystem`

This is necessary in order to have access to private members of
a `ConstraintSystem` for testing purposes, such as logic related
to potential binding computation.

roambotics/swift,apple/swift,benlangmuir/swift,jmgc/swift,parkera/swift,atrick/swift,hooman/swift,roambotics/swift,gregomni/swift,glessard/swift,rudkx/swift,xwu/swift,JGiola/swift,parkera/swift,JGiola/swift,parkera/swift,glessard/swift,jmgc/swift,jmgc/swift,rudkx/swift,JGiola/swift,hooman/swift,rudkx/swift,benlangmuir/swift,benlangmuir/swift,tkremenek/swift,jmgc/swift,apple/swift,hooman/swift,parkera/swift,atrick/swift,atrick/swift,hooman/swift,hooman/swift,gregomni/swift,parkera/swift,rudkx/swift,apple/swift,glessard/swift,tkremenek/swift,roambotics/swift,benlangmuir/swift,apple/swift,atrick/swift,xwu/swift,atrick/swift,ahoppen/swift,gregomni/swift,parkera/swift,glessard/swift,ahoppen/swift,benlangmuir/swift,ahoppen/swift,atrick/swift,xwu/swift,ahoppen/swift,gregomni/swift,apple/swift,glessard/swift,rudkx/swift,roambotics/swift,tkremenek/swift,jmgc/swift,jmgc/swift,JGiola/swift,JGiola/swift,roambotics/swift,tkremenek/swift,rudkx/swift,gregomni/swift,xwu/swift,tkremenek/swift,parkera/swift,xwu/swift,JGiola/swift,hooman/swift,glessard/swift,benlangmuir/swift,ahoppen/swift,tkremenek/swift,jmgc/swift,xwu/swift,gregomni/swift,tkremenek/swift,hooman/swift,roambotics/swift,apple/swift,ahoppen/swift,parkera/swift,xwu/swift