[ORC] Make MaterializationResponsibility immovable, pass by unique_ptr.

[ORC] Make MaterializationResponsibility immovable, pass by unique_ptr.

Making MaterializationResponsibility instances immovable allows their
associated VModuleKeys to be updated by the ExecutionSession while the
responsibility is still in-flight. This will be used in the upcoming
removable code feature to enable safe merging of resource keys even if
there are active compiles using the keys being merged.