Change PMTopLevelManager's PassManagers vector element type from

Pass* to PMDataManager*. PMDataManager is more specific than Pass,
so this more accurately describes the objects that are being stored.

This eliminates the need for several dynamic_casts to PMDataManager*.
It does introduce one dynamic_cast though, in dumpPasses(). Give
this one a comment describing why a dynamic_cast is being used.

llvm-svn: 48315