[clang][dataflow] Change the diagnoser API to receive a correctly typed lattice element

Previously, the diagnoser could only receive the Environment at a given program point. Now, it receives the complete dataflow state: the environment and lattice element.

This change does not contain any tests because we modify the checkDataflow function to rely on the newly introduced lattice element in PostVisitCFG, and existing tests that verify lattice elements depend on this overload of checkDataflow.

Reviewed By: gribozavr2, ymandel

Differential Revision: https://reviews.llvm.org/D139868