Handle inference variables in `CollectAllMismatches` correctly
1. Fix#106240
2. Treat int/float type variables correctly (see `src/test/ui/iterators/invalid-iterator-chain-with-int-infer.rs`), so we can point out things like "`Iterator::Item` changed to `{integer}` here"
Migrate mir_build diagnostics 2 of 3
The first three commits are fairly boring, however I've made some changes to the output of the match checking diagnostics.
