[clang][dataflow] Refactor `DataflowWorklist` types and add a WTO version.

Removes dependency of the DataflowWorklistBase type on the PostOrderCFGView. The
base had no particular connection to this order -- looks like it was included as
a convenience to the two derived classes rather than because it belonged there
conceptually. Removing it cleans up the base class a bit and makes it suitable
to extension with the new WTODataflowWorklist class.

Differential Revision: https://reviews.llvm.org/D153071