[Sema] Rework DiscardAssignmentExpr checking wrt SequenceExpr folding
[Sema] Rework DiscardAssignmentExpr checking wrt SequenceExpr folding

Now that we're checking DiscardAssignmentExprs in PreCheckExpr, we have to be careful to make sure we don't diagnose anything until after SequenceExprs have been folded, since the relevant AssignExprs won't even have a "right hand side" and "left hand side" until after folding.

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