fix(rustc_hir_analysis): handle Node::Synthetic in diagnostic_hir_wf_check

[DAstapov]

Closes #159557
Closes #148630

[rustbot]

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[rustbot]

This PR changes a file inside tests/crashes. If a crash was fixed, please move into the corresponding ui subdir and add 'Fixes #' to the PR description to autoclose the issue upon merge.

[jieyouxu]

This looks like #157856 next doors, where we are papering over a synthetic node being reachable here in HIR wfcheck 🤔 ICEing here seems more correct than trying to paper over the cases?

I can't dig into this any time soon (there's some non-trivial bootstrap reviews I need to prioritize) so rolling another compiler reviewer.

@rustbot reroll

View changes since this review

[folkertdev]

r? types

[lcnr]

There are currently no comments on Node::Synthetic and I don't have a good sense for when and how they are used. Would you mind trying to dig into the PRs which introduced that variant and why it's used. Both to ease the review and to more generally document this part of the code base?