[clang][dataflow] Modify `transfer` in `DataflowModel` to take `CFGElement` as...

[clang][dataflow] Modify `transfer` in `DataflowModel` to take `CFGElement` as input instead of `Stmt`.

To keep API of transfer functions consistent.

The single use of this transfer function in `ChromiumCheckModel` is also updated.

Reviewed By: gribozavr2, sgatev

Differential Revision: https://reviews.llvm.org/D133933