swift-api-checker.py: add -swift-only flag when checking ABI stability
swift-api-checker.py: add -swift-only flag when checking ABI stability

The existing logic of ABI checking only works for Swift symbols. We need to
add this flag so the script can work with Swift overlays.

rdar://60786484

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