Fix exception when mixing in roles that use `is built` at runtime
Fix exception when mixing in roles that use `is built` at runtime

In this case, Metamodel::BUILDPLAN.create_BUILDPLAN would attempt to
look up Mu through $*W when it no longer exists. Instead, HLLize a
VMNull to get it.

MasterDuke17/rakudo,labster/rakudo,rakudo/rakudo,tony-o/rakudo,nbrown/rakudo,salortiz/rakudo,rakudo/rakudo,rakudo/rakudo,salortiz/rakudo,tbrowder/rakudo,rakudo/rakudo,labster/rakudo,salortiz/rakudo,nbrown/rakudo,rakudo/rakudo,rakudo/rakudo,labster/rakudo,tbrowder/rakudo,salortiz/rakudo,softmoth/rakudo,softmoth/rakudo,tony-o/rakudo,nbrown/rakudo,labster/rakudo,tony-o/rakudo,tony-o/rakudo,labster/rakudo,tbrowder/rakudo,tony-o/rakudo,tbrowder/rakudo,softmoth/rakudo,softmoth/rakudo,tony-o/rakudo,MasterDuke17/rakudo,MasterDuke17/rakudo,nbrown/rakudo,salortiz/rakudo,tbrowder/rakudo,labster/rakudo,nbrown/rakudo,tbrowder/rakudo,MasterDuke17/rakudo,MasterDuke17/rakudo,MasterDuke17/rakudo,salortiz/rakudo,softmoth/rakudo,nbrown/rakudo