build: help pave the road for a mixed Python build
build: help pave the road for a mixed Python build

LLDB requires python3, however, gyb is a python2 tool.  Use the fact
that we have a newer CMake at our disposal and use the `Python2` package
in CMake to locate a viable python interpreter.  This allows us to have
references to both versions of python simultaneously.

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