Change PMTopLevelManager's PassManagers vector element type from

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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@48315 91177308-0d34-0410-b5e6-96231b3b80d8