`get_diagnostic_item` in a bunch more places (#15519)

For https://github.com/rust-lang/rust-clippy/issues/7784

The first commit is for all the places with rather trivial changes:
- replacing nested `is_diagnostic_item` `if`s with a match on
`get_diagnostic_item`
- storing the result of `get_diagnostic_item` in a variable and reusing
it

The rest of commits are for more involved changes, and for places where
changing to `get_diagnostic_item` allowed further simplifications -- in
the latter case, the follow-up commits are marked as `misc: `

This is roughly one evening's worth of changes, so I hope the amount
won't be overwhelming^^

changelog: none

0 files changed, 0 insertions, 0 deletions