[cmake] Do not override existing JOB_POOLS.
[cmake] Do not override existing JOB_POOLS.

When building Swift as part of an LLVM distribution the global property
JOB_POOLS would have been overriden by Swift. If one wanted parallel
linking in LLVM, the job pools setup by LLVM would have disappered.

This didn't happen in the standalone builds, because HandleLLVMOptions
is included in that case _after_ the JOB_POOLS property would have been
initialized without APPEND.

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