Serialization: Redo generic param xrefs to use GenericSignature
Serialization: Redo generic param xrefs to use GenericSignature

This code used GenericParamList::getOuterParameters() and
DeclContext::getGenericParamsOfContext(). The meaning of the
former is about to change, and the latter is going away.

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