utils/run-test: added lit's --filter option
utils/run-test: added lit's --filter option

Added a pass through of lit's `--filter` option to utils/run-test, to
allow running a subset of tests, for example `./utils/run-test
--filter=irgen` runs tests in `test/IRGen` and any other test with
"irgen" in its name.

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