[libSyntax] Allow declarations with external asm name to not have a body
[libSyntax] Allow declarations with external asm name to not have a body

This covers declarations of functions with the @_silgen_name attribute
used in the stdlib.

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