Add a safe default initializer for IRGenOptions::DWARFVersion.
Add a safe default initializer for IRGenOptions::DWARFVersion.

<rdar://problem/26472452>

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