build: cleanup some of the organization in `add_swift_host_library`
build: cleanup some of the organization in `add_swift_host_library`

This is just a cleanup of the implementation of
`add_swift_host_library`, reorganizing the flag computation and
application as well as the property setting.

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