build-script: support CMake install steps.
build-script: support CMake install steps.

Currently, bots that perfrom an installation will build LLDB once using CMake
because that is the default and then again using Xcode because the CMake
install step is not implemented by build-script. This patch adds CMake support
by calling the generic cmake install-lldb step.

<rdar://problem/51317901>

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