[NFC] Reorganize DiagnosticState::determineBehavior()
[NFC] Reorganize DiagnosticState::determineBehavior()

This makes it a little easier to reason abut what’s going on while also fixing a small, currently-unexercisable flaw that would have previously occurred if you used limitBehavior(DiagnosticBehavior::Warning) and emitted a remark.

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