IRGen: Use correct generic sig when mangling opaque underlying type.
IRGen: Use correct generic sig when mangling opaque underlying type.

Caught while testing on Swift 5.1.

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