Pass the right HIR back from `get_fn_decl`
Fixes#109232
Makes sure that the `fn_id: HirId` that we pass to `suggest_missing_return_type` matches up with the `fn_decl: hir::FnDecl` that we pass to it, so the late-bound vars that we fetch from the former match up with the types in the latter...
This HIR suggestion code really needs a big refactor. I've tried to do it in the past (a couple of attempts), but it's a super tangled mess. It really shouldn't be passing around things like `hir::Node` and just deal with `LocalDefId`s everywhere... Anyways, I'd rather fix this ICE, now.
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