[NFC] Give NullablePtr some PointerLikeTypeTraits
[NFC] Give NullablePtr some PointerLikeTypeTraits

Allows it to be used in PointerIntPair and PointerUnion.

xwu/swift,apple/swift,rudkx/swift,tkremenek/swift,parkera/swift,atrick/swift,jmgc/swift,atrick/swift,roambotics/swift,hooman/swift,xwu/swift,JGiola/swift,jmgc/swift,atrick/swift,hooman/swift,ahoppen/swift,ahoppen/swift,xwu/swift,benlangmuir/swift,parkera/swift,parkera/swift,gregomni/swift,tkremenek/swift,benlangmuir/swift,glessard/swift,JGiola/swift,hooman/swift,parkera/swift,ahoppen/swift,gregomni/swift,jmgc/swift,benlangmuir/swift,roambotics/swift,hooman/swift,JGiola/swift,apple/swift,rudkx/swift,roambotics/swift,roambotics/swift,jmgc/swift,atrick/swift,jmgc/swift,atrick/swift,rudkx/swift,apple/swift,JGiola/swift,parkera/swift,xwu/swift,benlangmuir/swift,benlangmuir/swift,glessard/swift,jmgc/swift,jmgc/swift,roambotics/swift,xwu/swift,apple/swift,rudkx/swift,glessard/swift,benlangmuir/swift,gregomni/swift,gregomni/swift,gregomni/swift,parkera/swift,glessard/swift,glessard/swift,tkremenek/swift,hooman/swift,tkremenek/swift,ahoppen/swift,parkera/swift,xwu/swift,ahoppen/swift,hooman/swift,glessard/swift,tkremenek/swift,apple/swift,tkremenek/swift,tkremenek/swift,JGiola/swift,rudkx/swift,ahoppen/swift,JGiola/swift,apple/swift,atrick/swift,xwu/swift,rudkx/swift,gregomni/swift,hooman/swift,roambotics/swift,parkera/swift