[build-script] Removed the always-broken support for building PlaygroundLogger on Linux.
[build-script] Removed the always-broken support for building PlaygroundLogger on Linux.

As far as I can tell, this never worked, so removing it cannot break anything.
Now PlaygroundLogger is treated similarly to PlaygroundSupport, directly referencing xcodebuild instead of indirecting through variables.
This commit also introduces explicit error messages when attempting to build PlaygroundLogger or PlaygroundSupport for non-Darwin platforms.

This addresses <rdar://problem/36594779>.

tkremenek/swift,jmgc/swift,lorentey/swift,xedin/swift,jckarter/swift,austinzheng/swift,karwa/swift,allevato/swift,jmgc/swift,roambotics/swift,gribozavr/swift,JGiola/swift,JGiola/swift,rudkx/swift,jopamer/swift,aschwaighofer/swift,glessard/swift,CodaFi/swift,practicalswift/swift,airspeedswift/swift,CodaFi/swift,huonw/swift,hooman/swift,jckarter/swift,harlanhaskins/swift,jckarter/swift,jmgc/swift,parkera/swift,tjw/swift,jopamer/swift,sschiau/swift,brentdax/swift,stephentyrone/swift,karwa/swift,stephentyrone/swift,alblue/swift,atrick/swift,nathawes/swift,allevato/swift,tjw/swift,amraboelela/swift,jmgc/swift,benlangmuir/swift,danielmartin/swift,shahmishal/swift,devincoughlin/swift,danielmartin/swift,jopamer/swift,amraboelela/swift,practicalswift/swift,huonw/swift,huonw/swift,sschiau/swift,apple/swift,huonw/swift,gribozavr/swift,atrick/swift,natecook1000/swift,airspeedswift/swift,natecook1000/swift,austinzheng/swift,gribozavr/swift,atrick/swift,jopamer/swift,parkera/swift,jmgc/swift,tkremenek/swift,natecook1000/swift,atrick/swift,airspeedswift/swift,aschwaighofer/swift,shajrawi/swift,stephentyrone/swift,tjw/swift,hooman/swift,tkremenek/swift,danielmartin/swift,glessard/swift,airspeedswift/swift,hooman/swift,ahoppen/swift,glessard/swift,austinzheng/swift,tjw/swift,brentdax/swift,apple/swift,harlanhaskins/swift,alblue/swift,shajrawi/swift,parkera/swift,benlangmuir/swift,ahoppen/swift,danielmartin/swift,natecook1000/swift,shahmishal/swift,gregomni/swift,CodaFi/swift,huonw/swift,aschwaighofer/swift,jckarter/swift,devincoughlin/swift,atrick/swift,hooman/swift,xwu/swift,xwu/swift,atrick/swift,gribozavr/swift,austinzheng/swift,benlangmuir/swift,tjw/swift,allevato/swift,airspeedswift/swift,karwa/swift,stephentyrone/swift,alblue/swift,lorentey/swift,gregomni/swift,glessard/swift,jckarter/swift,apple/swift,apple/swift,ahoppen/swift,zisko/swift,huonw/swift,aschwaighofer/swift,CodaFi/swift,apple/swift,karwa/swift,zisko/swift,lorentey/swift,CodaFi/swift,zisko/swift,alblue/swift,allevato/swift,ahoppen/swift,devincoughlin/swift,jmgc/swift,shajrawi/swift,lorentey/swift,nathawes/swift,lorentey/swift,shahmishal/swift,gregomni/swift,benlangmuir/swift,harlanhaskins/swift,xedin/swift,devincoughlin/swift,glessard/swift,lorentey/swift,gregomni/swift,glessard/swift,parkera/swift,alblue/swift,brentdax/swift,devincoughlin/swift,brentdax/swift,zisko/swift,practicalswift/swift,gribozavr/swift,shajrawi/swift,ahoppen/swift,xwu/swift,gregomni/swift,parkera/swift,natecook1000/swift,stephentyrone/swift,lorentey/swift,austinzheng/swift,gribozavr/swift,roambotics/swift,airspeedswift/swift,danielmartin/swift,amraboelela/swift,jckarter/swift,rudkx/swift,xedin/swift,shajrawi/swift,aschwaighofer/swift,airspeedswift/swift,harlanhaskins/swift,karwa/swift,sschiau/swift,JGiola/swift,jmgc/swift,shajrawi/swift,roambotics/swift,hooman/swift,allevato/swift,sschiau/swift,alblue/swift,stephentyrone/swift,sschiau/swift,nathawes/swift,xedin/swift,parkera/swift,nathawes/swift,rudkx/swift,lorentey/swift,shahmishal/swift,practicalswift/swift,ahoppen/swift,practicalswift/swift,devincoughlin/swift,tjw/swift,CodaFi/swift,shajrawi/swift,rudkx/swift,aschwaighofer/swift,roambotics/swift,alblue/swift,practicalswift/swift,zisko/swift,shahmishal/swift,huonw/swift,stephentyrone/swift,tkremenek/swift,gregomni/swift,JGiola/swift,brentdax/swift,shajrawi/swift,karwa/swift,jopamer/swift,JGiola/swift,amraboelela/swift,shahmishal/swift,shahmishal/swift,austinzheng/swift,danielmartin/swift,natecook1000/swift,tjw/swift,xwu/swift,harlanhaskins/swift,rudkx/swift,practicalswift/swift,harlanhaskins/swift,practicalswift/swift,nathawes/swift,zisko/swift,rudkx/swift,sschiau/swift,austinzheng/swift,amraboelela/swift,danielmartin/swift,roambotics/swift,parkera/swift,xwu/swift,xedin/swift,harlanhaskins/swift,roambotics/swift,nathawes/swift,jopamer/swift,gribozavr/swift,brentdax/swift,aschwaighofer/swift,shahmishal/swift,amraboelela/swift,benlangmuir/swift,xwu/swift,amraboelela/swift,karwa/swift,xedin/swift,apple/swift,tkremenek/swift,zisko/swift,devincoughlin/swift,hooman/swift,devincoughlin/swift,CodaFi/swift,hooman/swift,benlangmuir/swift,parkera/swift,xedin/swift,jckarter/swift,brentdax/swift,nathawes/swift,jopamer/swift,sschiau/swift,natecook1000/swift,xedin/swift,xwu/swift,sschiau/swift,karwa/swift,allevato/swift,allevato/swift,tkremenek/swift,JGiola/swift,tkremenek/swift,gribozavr/swift