Rollup merge of #145080 - ashivaram23:graphviz_escape, r=lqd

Escape diff strings in MIR dataflow graphviz

This uses `dot::escape_html` on diff strings so that `<` and `>` are properly escaped in MIR dataflow graphviz.

Previously the output could be malformed. For example, the graphviz for borrow check `MaybeInitializedPlaces` for the example in rust-lang/rust#145032 contained an unescaped `Fork<'_>`.

Author: Zalathar