[clang][dataflow] Make the type of the post visit callback consistent

[clang][dataflow] Make the type of the post visit callback consistent

Make the types of the post visit callbacks in `transferBlock` and
`runTypeErasedDataflowAnalysis` consistent.

Differential Revision: https://reviews.llvm.org/D131014

Reviewed-by: ymandel, xazax.hun, gribozavr2