[Type checker] Lazily synthesize the empty implicit destructor of a class.
[Type checker] Lazily synthesize the empty implicit destructor of a class.

This is more a matter of principle than an optimization, eliminating
unnecessary calls to `AbstractFunctionDecl::setBody()`.

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