Handle multiple @available attributes correctly
Handle multiple @available attributes correctly

The previous commit broke a promise made by the implementation it replaced: if there were two or more @available(introduced:) attributes for the same platform, the greatest version wins. This commit restores that property while still completely overriding the parent platform’s availability with a child platform’s.

airspeedswift/swift,JGiola/swift,harlanhaskins/swift,roambotics/swift,atrick/swift,parkera/swift,harlanhaskins/swift,roambotics/swift,ahoppen/swift,roambotics/swift,stephentyrone/swift,ahoppen/swift,JGiola/swift,xwu/swift,benlangmuir/swift,atrick/swift,tkremenek/swift,xwu/swift,stephentyrone/swift,ahoppen/swift,aschwaighofer/swift,parkera/swift,hooman/swift,harlanhaskins/swift,aschwaighofer/swift,benlangmuir/swift,allevato/swift,allevato/swift,jmgc/swift,hooman/swift,nathawes/swift,aschwaighofer/swift,JGiola/swift,tkremenek/swift,allevato/swift,rudkx/swift,hooman/swift,stephentyrone/swift,jmgc/swift,allevato/swift,jmgc/swift,glessard/swift,harlanhaskins/swift,aschwaighofer/swift,jckarter/swift,roambotics/swift,aschwaighofer/swift,airspeedswift/swift,nathawes/swift,ahoppen/swift,airspeedswift/swift,hooman/swift,rudkx/swift,glessard/swift,xwu/swift,JGiola/swift,apple/swift,jckarter/swift,airspeedswift/swift,nathawes/swift,airspeedswift/swift,xwu/swift,stephentyrone/swift,JGiola/swift,harlanhaskins/swift,airspeedswift/swift,harlanhaskins/swift,jckarter/swift,rudkx/swift,parkera/swift,CodaFi/swift,rudkx/swift,tkremenek/swift,xwu/swift,xwu/swift,hooman/swift,parkera/swift,CodaFi/swift,xwu/swift,parkera/swift,CodaFi/swift,atrick/swift,jmgc/swift,atrick/swift,aschwaighofer/swift,nathawes/swift,CodaFi/swift,airspeedswift/swift,stephentyrone/swift,apple/swift,gregomni/swift,apple/swift,nathawes/swift,benlangmuir/swift,tkremenek/swift,harlanhaskins/swift,glessard/swift,jmgc/swift,gregomni/swift,CodaFi/swift,benlangmuir/swift,jckarter/swift,allevato/swift,apple/swift,jckarter/swift,parkera/swift,roambotics/swift,allevato/swift,atrick/swift,ahoppen/swift,benlangmuir/swift,nathawes/swift,jmgc/swift,apple/swift,CodaFi/swift,benlangmuir/swift,rudkx/swift,glessard/swift,CodaFi/swift,atrick/swift,jckarter/swift,JGiola/swift,gregomni/swift,roambotics/swift,parkera/swift,allevato/swift,gregomni/swift,gregomni/swift,nathawes/swift,tkremenek/swift,glessard/swift,hooman/swift,gregomni/swift,parkera/swift,jckarter/swift,glessard/swift,rudkx/swift,aschwaighofer/swift,stephentyrone/swift,stephentyrone/swift,jmgc/swift,apple/swift,tkremenek/swift,ahoppen/swift,hooman/swift,tkremenek/swift