[CodeCompletion] Don't throw away parsed return type for function decls
[CodeCompletion] Don't throw away parsed return type for function decls

Parsed type should be in the result AST to maintain the source range of
the decl.

rdar://problem/65906026

allevato/swift,tkremenek/swift,atrick/swift,apple/swift,ahoppen/swift,parkera/swift,hooman/swift,airspeedswift/swift,apple/swift,rudkx/swift,xwu/swift,glessard/swift,rudkx/swift,rudkx/swift,xwu/swift,airspeedswift/swift,gregomni/swift,ahoppen/swift,jmgc/swift,nathawes/swift,rudkx/swift,airspeedswift/swift,glessard/swift,gregomni/swift,tkremenek/swift,parkera/swift,airspeedswift/swift,parkera/swift,benlangmuir/swift,nathawes/swift,allevato/swift,jckarter/swift,jckarter/swift,gregomni/swift,nathawes/swift,apple/swift,parkera/swift,hooman/swift,jmgc/swift,xwu/swift,jckarter/swift,glessard/swift,jckarter/swift,airspeedswift/swift,ahoppen/swift,xwu/swift,apple/swift,airspeedswift/swift,jmgc/swift,parkera/swift,hooman/swift,JGiola/swift,benlangmuir/swift,roambotics/swift,roambotics/swift,apple/swift,gregomni/swift,allevato/swift,hooman/swift,CodaFi/swift,atrick/swift,airspeedswift/swift,roambotics/swift,roambotics/swift,JGiola/swift,benlangmuir/swift,CodaFi/swift,JGiola/swift,tkremenek/swift,glessard/swift,CodaFi/swift,hooman/swift,xwu/swift,atrick/swift,allevato/swift,roambotics/swift,tkremenek/swift,tkremenek/swift,glessard/swift,benlangmuir/swift,allevato/swift,rudkx/swift,benlangmuir/swift,rudkx/swift,tkremenek/swift,jmgc/swift,ahoppen/swift,jckarter/swift,JGiola/swift,tkremenek/swift,atrick/swift,CodaFi/swift,CodaFi/swift,jmgc/swift,glessard/swift,nathawes/swift,JGiola/swift,JGiola/swift,hooman/swift,nathawes/swift,benlangmuir/swift,CodaFi/swift,parkera/swift,ahoppen/swift,xwu/swift,atrick/swift,jmgc/swift,jckarter/swift,parkera/swift,allevato/swift,parkera/swift,xwu/swift,allevato/swift,nathawes/swift,roambotics/swift,gregomni/swift,jckarter/swift,ahoppen/swift,gregomni/swift,nathawes/swift,apple/swift,jmgc/swift,atrick/swift,hooman/swift,CodaFi/swift