unittests: add a hack to use the host compiler
unittests: add a hack to use the host compiler

Unfortunately, ASAN breaks with the just built compiler.  The runtime
and the runtime tests should really use the same compiler.  As a
workaround, if the host compiler is clang, just use that for the time
being.  This should fix the build on the ASAN bots.

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