tests: disable IRGen/existential_shape_metadata.swift
tests: disable IRGen/existential_shape_metadata.swift

to unblock CI

benlangmuir/swift,JGiola/swift,gregomni/swift,atrick/swift,JGiola/swift,apple/swift,apple/swift,gregomni/swift,apple/swift,glessard/swift,apple/swift,atrick/swift,roambotics/swift,benlangmuir/swift,JGiola/swift,ahoppen/swift,ahoppen/swift,ahoppen/swift,JGiola/swift,atrick/swift,apple/swift,JGiola/swift,benlangmuir/swift,roambotics/swift,benlangmuir/swift,glessard/swift,atrick/swift,atrick/swift,roambotics/swift,JGiola/swift,glessard/swift,roambotics/swift,atrick/swift,ahoppen/swift,ahoppen/swift,roambotics/swift,glessard/swift,apple/swift,glessard/swift,roambotics/swift,glessard/swift,gregomni/swift,gregomni/swift,ahoppen/swift,gregomni/swift,benlangmuir/swift,benlangmuir/swift,gregomni/swift