[update-checkout] Add new branch configs for Swift 5.1
[update-checkout] Add new branch configs for Swift 5.1

Add a tentative branch config for "swift-5.1-branch". Not all of the repos
are branched yet. I went ahead and added a reference to swift-5.1-branch for
the Swift repo, so that I can use this to start pulling together something
with cherry-picks from master-next. (The current master-next branch has
moved past the point where we branched the LLVM repos for swift-5.1-branch,
so we can't just grab a copy of master-next.) I also added a separate config
for "swift-5.1-old-llvm-branch", which will be used temporarily to track the
same Swift content as swift-5.1-branch but still building with the version
of LLVM in swift-5.0-branch.

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