[ORC] In defineMaterializing, error out early if tracker is defunct.

An in-flight materialization may try to claim responsibility for new symbols
(via MaterializationResponsibility::defineMaterializing) after the tracker that
is associated with the materialization is removed, leaving the tracker defunct.

Failure to error out early here could leave the JITDylib in an invalid state,
with defineMaterializing associating new symbols with the already-defunct
tracker. Erroring out early prevents this.