disable wipe, add clean-llvm, fix platform_snap_slaves
disable wipe, add clean-llvm, fix platform_snap_slaves

rust-lang-deprecated/rust-buildbot,rust-lang/rust-buildbot,rust-lang-deprecated/rust-buildbot,aidanhs/rust-buildbot,rust-lang/rust-buildbot,edunham/rust-buildbot,edunham/rust-buildbot,rust-lang/rust-buildbot,aidanhs/rust-buildbot,aidanhs/rust-buildbot,edunham/rust-buildbot,rust-lang-deprecated/rust-buildbot