[Runtime] Avoid function pointer indirection in refcounting functions.
[Runtime] Avoid function pointer indirection in refcounting functions.

Functions like swift_retain call through a function pointer so that Instruments can interpose. This slows down the common case where there is no interposition. Instead, initialize the function pointers to NULL and call through directly to the real implementation when it's NULL. The compiler is smart enough to inline this call and the result is a single conditional branch rather than a function pointer call.

rdar://problem/18307425
parkera/swift,hooman/swift,hooman/swift,devincoughlin/swift,harlanhaskins/swift,devincoughlin/swift,apple/swift,gregomni/swift,JGiola/swift,JGiola/swift,airspeedswift/swift,jmgc/swift,apple/swift,ahoppen/swift,devincoughlin/swift,ahoppen/swift,rudkx/swift,nathawes/swift,gregomni/swift,jckarter/swift,lorentey/swift,apple/swift,xwu/swift,karwa/swift,atrick/swift,JGiola/swift,parkera/swift,ahoppen/swift,roambotics/swift,parkera/swift,karwa/swift,jmgc/swift,hooman/swift,tkremenek/swift,rudkx/swift,nathawes/swift,lorentey/swift,sschiau/swift,rudkx/swift,rudkx/swift,allevato/swift,CodaFi/swift,xwu/swift,roambotics/swift,apple/swift,glessard/swift,gribozavr/swift,parkera/swift,roambotics/swift,CodaFi/swift,harlanhaskins/swift,allevato/swift,CodaFi/swift,tkremenek/swift,gregomni/swift,devincoughlin/swift,xwu/swift,harlanhaskins/swift,jckarter/swift,nathawes/swift,hooman/swift,gribozavr/swift,aschwaighofer/swift,allevato/swift,glessard/swift,tkremenek/swift,karwa/swift,JGiola/swift,gregomni/swift,lorentey/swift,aschwaighofer/swift,xwu/swift,nathawes/swift,devincoughlin/swift,aschwaighofer/swift,gribozavr/swift,xwu/swift,allevato/swift,tkremenek/swift,harlanhaskins/swift,airspeedswift/swift,aschwaighofer/swift,gribozavr/swift,xwu/swift,ahoppen/swift,devincoughlin/swift,airspeedswift/swift,rudkx/swift,lorentey/swift,glessard/swift,devincoughlin/swift,jmgc/swift,sschiau/swift,hooman/swift,parkera/swift,atrick/swift,lorentey/swift,stephentyrone/swift,hooman/swift,glessard/swift,nathawes/swift,apple/swift,roambotics/swift,jckarter/swift,JGiola/swift,sschiau/swift,gribozavr/swift,gregomni/swift,gribozavr/swift,gribozavr/swift,parkera/swift,stephentyrone/swift,ahoppen/swift,sschiau/swift,lorentey/swift,tkremenek/swift,airspeedswift/swift,tkremenek/swift,jmgc/swift,lorentey/swift,allevato/swift,sschiau/swift,allevato/swift,jmgc/swift,sschiau/swift,jckarter/swift,karwa/swift,stephentyrone/swift,atrick/swift,karwa/swift,roambotics/swift,nathawes/swift,hooman/swift,CodaFi/swift,atrick/swift,airspeedswift/swift,glessard/swift,CodaFi/swift,harlanhaskins/swift,parkera/swift,karwa/swift,aschwaighofer/swift,karwa/swift,rudkx/swift,stephentyrone/swift,benlangmuir/swift,tkremenek/swift,harlanhaskins/swift,jmgc/swift,airspeedswift/swift,benlangmuir/swift,CodaFi/swift,jckarter/swift,airspeedswift/swift,parkera/swift,jckarter/swift,atrick/swift,gregomni/swift,apple/swift,sschiau/swift,harlanhaskins/swift,nathawes/swift,xwu/swift,glessard/swift,gribozavr/swift,allevato/swift,jckarter/swift,jmgc/swift,benlangmuir/swift,benlangmuir/swift,ahoppen/swift,aschwaighofer/swift,stephentyrone/swift,benlangmuir/swift,JGiola/swift,devincoughlin/swift,stephentyrone/swift,roambotics/swift,lorentey/swift,CodaFi/swift,karwa/swift,stephentyrone/swift,atrick/swift,aschwaighofer/swift,benlangmuir/swift,sschiau/swift