[clang][dataflow] Identify post-visit state changes in the HTML logger. (#66746)

Previously, post-visit state changes were indistinguishable from
ordinary
iterations, which could give a confusing picture of how many iterations
a block
needs to converge.

Now, post-visit state changes are marked with "post-visit" instead of an
iteration number:


![screenshot](https://github.com/llvm/llvm-project/assets/29098113/5e9553d6-dfaa-45d3-8ea4-e623a14ee4c5))