[silgen] prepareApplyExpr => CallEmission::forApplyExpr.
[silgen] prepareApplyExpr => CallEmission::forApplyExpr.

prepareApplyExpr is a static function that changes an expr into a CallEmission
and has no other uses beyond that purpose. This is the definition of a static
factory method. This PR performs that refactor.

NFC.

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