[next] Do not use libcxx standalone build
[next] Do not use libcxx standalone build

This was removed in llvm/llvm-project
6f17768e11480063f4c2bcbeea559505fee3ea19. Use new build instead.

roambotics/swift,glessard/swift,apple/swift,roambotics/swift,benlangmuir/swift,glessard/swift,apple/swift,glessard/swift,apple/swift,apple/swift,benlangmuir/swift,benlangmuir/swift,glessard/swift,apple/swift,roambotics/swift,glessard/swift,roambotics/swift,benlangmuir/swift,roambotics/swift,roambotics/swift,benlangmuir/swift,benlangmuir/swift,apple/swift,glessard/swift