[Index] Don't report extensions with nothing to index in system modules
[Index] Don't report extensions with nothing to index in system modules

The indexer was looking into “empty” extensions to public types,
triggering computing their USR which could fail at
deserializing the implementation-only imported types. As a solution,
don’t index extensions with nothing to index in system modules.

rdar://70225906

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