Don't change the token kind of `yield` until we commit to a statement.
Don't change the token kind of `yield` until we commit to a statement.

Per Rintaro's code review.

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