[cmpcodesize] Assert env vars set properly
[cmpcodesize] Assert env vars set properly

`$SWIFT_OLD_BUILDDIR` and `$SWIFT_NEW_BUILDDIR` are only necessary when
not using `--` to delineate two sets of files.

1. Only grab them from the environment when `--` is *not* used.
2. Assert if they're not present in that codepath. Print what *is*
   specified in the environment, in order to help the user correct
   their mistake.

manavgabhawala/swift,russbishop/swift,alblue/swift,airspeedswift/swift,djwbrown/swift,practicalswift/swift,brentdax/swift,manavgabhawala/swift,Jnosh/swift,danielmartin/swift,JaSpa/swift,Jnosh/swift,Jnosh/swift,apple/swift,arvedviehweger/swift,CodaFi/swift,modocache/swift,xwu/swift,milseman/swift,practicalswift/swift,gmilos/swift,stephentyrone/swift,alblue/swift,kstaring/swift,IngmarStein/swift,gribozavr/swift,kperryua/swift,stephentyrone/swift,harlanhaskins/swift,austinzheng/swift,OscarSwanros/swift,calebd/swift,JGiola/swift,stephentyrone/swift,tinysun212/swift-windows,hooman/swift,KrishMunot/swift,benlangmuir/swift,jckarter/swift,JGiola/swift,ahoppen/swift,shajrawi/swift,therealbnut/swift,ken0nek/swift,jmgc/swift,milseman/swift,JaSpa/swift,modocache/swift,slavapestov/swift,codestergit/swift,huonw/swift,gregomni/swift,codestergit/swift,uasys/swift,kperryua/swift,zisko/swift,apple/swift,shajrawi/swift,jopamer/swift,hooman/swift,nathawes/swift,dduan/swift,felix91gr/swift,shajrawi/swift,alblue/swift,kstaring/swift,frootloops/swift,xwu/swift,ahoppen/swift,lorentey/swift,xwu/swift,jopamer/swift,parkera/swift,jtbandes/swift,russbishop/swift,allevato/swift,hughbe/swift,lorentey/swift,gmilos/swift,frootloops/swift,practicalswift/swift,therealbnut/swift,gottesmm/swift,shahmishal/swift,kstaring/swift,jopamer/swift,therealbnut/swift,adrfer/swift,tkremenek/swift,slavapestov/swift,bitjammer/swift,uasys/swift,benlangmuir/swift,devincoughlin/swift,OscarSwanros/swift,practicalswift/swift,glessard/swift,deyton/swift,johnno1962d/swift,tjw/swift,SwiftAndroid/swift,aschwaighofer/swift,glessard/swift,shajrawi/swift,tardieu/swift,dduan/swift,IngmarStein/swift,bitjammer/swift,natecook1000/swift,johnno1962d/swift,devincoughlin/swift,amraboelela/swift,harlanhaskins/swift,return/swift,uasys/swift,tjw/swift,johnno1962d/swift,adrfer/swift,CodaFi/swift,OscarSwanros/swift,Jnosh/swift,allevato/swift,glessard/swift,aschwaighofer/swift,xedin/swift,gribozavr/swift,allevato/swift,harlanhaskins/swift,uasys/swift,karwa/swift,gmilos/swift,bitjammer/swift,jckarter/swift,zisko/swift,airspeedswift/swift,apple/swift,tkremenek/swift,jckarter/swift,shajrawi/swift,brentdax/swift,natecook1000/swift,Jnosh/swift,shajrawi/swift,bitjammer/swift,apple/swift,brentdax/swift,ben-ng/swift,hooman/swift,lorentey/swift,huonw/swift,karwa/swift,nathawes/swift,xedin/swift,IngmarStein/swift,austinzheng/swift,KrishMunot/swift,jmgc/swift,codestergit/swift,danielmartin/swift,SwiftAndroid/swift,JaSpa/swift,stephentyrone/swift,aschwaighofer/swift,gribozavr/swift,danielmartin/swift,karwa/swift,felix91gr/swift,nathawes/swift,gmilos/swift,huonw/swift,manavgabhawala/swift,dduan/swift,rudkx/swift,jckarter/swift,milseman/swift,dduan/swift,natecook1000/swift,johnno1962d/swift,ahoppen/swift,jmgc/swift,tkremenek/swift,ben-ng/swift,ben-ng/swift,dreamsxin/swift,hooman/swift,brentdax/swift,dduan/swift,return/swift,adrfer/swift,devincoughlin/swift,parkera/swift,ken0nek/swift,uasys/swift,tinysun212/swift-windows,jtbandes/swift,arvedviehweger/swift,gmilos/swift,KrishMunot/swift,swiftix/swift,amraboelela/swift,bitjammer/swift,tinysun212/swift-windows,bitjammer/swift,rudkx/swift,jopamer/swift,atrick/swift,stephentyrone/swift,dduan/swift,manavgabhawala/swift,JGiola/swift,CodaFi/swift,apple/swift,adrfer/swift,alblue/swift,gregomni/swift,calebd/swift,IngmarStein/swift,hughbe/swift,therealbnut/swift,roambotics/swift,johnno1962d/swift,xedin/swift,natecook1000/swift,brentdax/swift,benlangmuir/swift,nathawes/swift,tardieu/swift,amraboelela/swift,atrick/swift,JGiola/swift,airspeedswift/swift,arvedviehweger/swift,sschiau/swift,tardieu/swift,milseman/swift,CodaFi/swift,xedin/swift,felix91gr/swift,xedin/swift,tinysun212/swift-windows,JaSpa/swift,alblue/swift,return/swift,slavapestov/swift,CodaFi/swift,parkera/swift,frootloops/swift,tkremenek/swift,airspeedswift/swift,codestergit/swift,ken0nek/swift,tjw/swift,OscarSwanros/swift,danielmartin/swift,karwa/swift,tinysun212/swift-windows,hooman/swift,SwiftAndroid/swift,benlangmuir/swift,benlangmuir/swift,roambotics/swift,jtbandes/swift,russbishop/swift,adrfer/swift,danielmartin/swift,tinysun212/swift-windows,gregomni/swift,xedin/swift,SwiftAndroid/swift,jtbandes/swift,harlanhaskins/swift,natecook1000/swift,sschiau/swift,felix91gr/swift,atrick/swift,kperryua/swift,tjw/swift,amraboelela/swift,huonw/swift,aschwaighofer/swift,KrishMunot/swift,karwa/swift,return/swift,uasys/swift,ken0nek/swift,airspeedswift/swift,lorentey/swift,KrishMunot/swift,practicalswift/swift,roambotics/swift,kperryua/swift,shahmishal/swift,allevato/swift,devincoughlin/swift,milseman/swift,devincoughlin/swift,harlanhaskins/swift,calebd/swift,aschwaighofer/swift,modocache/swift,glessard/swift,shahmishal/swift,frootloops/swift,lorentey/swift,JGiola/swift,huonw/swift,deyton/swift,frootloops/swift,austinzheng/swift,johnno1962d/swift,ken0nek/swift,tkremenek/swift,jtbandes/swift,jmgc/swift,devincoughlin/swift,natecook1000/swift,xedin/swift,amraboelela/swift,felix91gr/swift,shajrawi/swift,austinzheng/swift,IngmarStein/swift,tinysun212/swift-windows,sschiau/swift,ben-ng/swift,deyton/swift,calebd/swift,milseman/swift,djwbrown/swift,deyton/swift,OscarSwanros/swift,allevato/swift,sschiau/swift,tardieu/swift,harlanhaskins/swift,ben-ng/swift,sschiau/swift,practicalswift/swift,ken0nek/swift,natecook1000/swift,huonw/swift,gribozavr/swift,hughbe/swift,amraboelela/swift,OscarSwanros/swift,arvedviehweger/swift,parkera/swift,jopamer/swift,adrfer/swift,brentdax/swift,JGiola/swift,nathawes/swift,deyton/swift,xedin/swift,alblue/swift,slavapestov/swift,jmgc/swift,nathawes/swift,apple/swift,gmilos/swift,hughbe/swift,russbishop/swift,JaSpa/swift,slavapestov/swift,lorentey/swift,arvedviehweger/swift,rudkx/swift,frootloops/swift,sschiau/swift,return/swift,kstaring/swift,xwu/swift,JaSpa/swift,hughbe/swift,atrick/swift,jopamer/swift,Jnosh/swift,slavapestov/swift,zisko/swift,shahmishal/swift,kstaring/swift,ahoppen/swift,russbishop/swift,tkremenek/swift,gregomni/swift,gottesmm/swift,glessard/swift,gottesmm/swift,kperryua/swift,codestergit/swift,slavapestov/swift,modocache/swift,OscarSwanros/swift,jmgc/swift,zisko/swift,glessard/swift,alblue/swift,manavgabhawala/swift,parkera/swift,tardieu/swift,russbishop/swift,jckarter/swift,swiftix/swift,rudkx/swift,ben-ng/swift,zisko/swift,JaSpa/swift,deyton/swift,Jnosh/swift,benlangmuir/swift,airspeedswift/swift,bitjammer/swift,xwu/swift,harlanhaskins/swift,return/swift,danielmartin/swift,nathawes/swift,swiftix/swift,roambotics/swift,roambotics/swift,shahmishal/swift,therealbnut/swift,dduan/swift,deyton/swift,parkera/swift,practicalswift/swift,therealbnut/swift,modocache/swift,brentdax/swift,jmgc/swift,johnno1962d/swift,CodaFi/swift,allevato/swift,hughbe/swift,aschwaighofer/swift,gottesmm/swift,calebd/swift,gribozavr/swift,CodaFi/swift,karwa/swift,return/swift,hooman/swift,kperryua/swift,milseman/swift,huonw/swift,uasys/swift,KrishMunot/swift,gribozavr/swift,dreamsxin/swift,allevato/swift,sschiau/swift,austinzheng/swift,therealbnut/swift,jopamer/swift,roambotics/swift,jckarter/swift,swiftix/swift,tjw/swift,arvedviehweger/swift,xwu/swift,amraboelela/swift,parkera/swift,frootloops/swift,felix91gr/swift,calebd/swift,shajrawi/swift,karwa/swift,practicalswift/swift,devincoughlin/swift,tjw/swift,rudkx/swift,gmilos/swift,calebd/swift,kstaring/swift,russbishop/swift,adrfer/swift,stephentyrone/swift,kperryua/swift,arvedviehweger/swift,codestergit/swift,modocache/swift,gottesmm/swift,swiftix/swift,gregomni/swift,aschwaighofer/swift,tardieu/swift,atrick/swift,ken0nek/swift,djwbrown/swift,tardieu/swift,stephentyrone/swift,lorentey/swift,ahoppen/swift,atrick/swift,devincoughlin/swift,gregomni/swift,jtbandes/swift,jtbandes/swift,manavgabhawala/swift,shahmishal/swift,KrishMunot/swift,SwiftAndroid/swift,IngmarStein/swift,austinzheng/swift,gribozavr/swift,codestergit/swift,felix91gr/swift,SwiftAndroid/swift,rudkx/swift,danielmartin/swift,zisko/swift,karwa/swift,kstaring/swift,gottesmm/swift,zisko/swift,swiftix/swift,jckarter/swift,djwbrown/swift,SwiftAndroid/swift,austinzheng/swift,modocache/swift,hughbe/swift,manavgabhawala/swift,sschiau/swift,djwbrown/swift,lorentey/swift,ahoppen/swift,shahmishal/swift,tkremenek/swift,gribozavr/swift,hooman/swift,djwbrown/swift,airspeedswift/swift,shahmishal/swift,gottesmm/swift,IngmarStein/swift,tjw/swift,xwu/swift,swiftix/swift,djwbrown/swift,ben-ng/swift,parkera/swift