[Runtime] Accept nil in swift_getObjCClassFromMetadata.
[Runtime] Accept nil in swift_getObjCClassFromMetadata.

Somehow, clang was generating code that accepted nil and returned nil in swift_getObjCClassFromMetadata, but is no longer doing so. Some code relies on this to work, so switch to dyn_cast_or_null to accept nil explicitly.

rdar://74895271

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