Fix ICE \Unexpected node Synthetic\ in diagnostic_hir_wf_check

[Ijtihed]

Fixes #148630

diagnostic_hir_wf_check panics on Node::Synthetic when walking HIR types for WF error span refinement. RPITIT opaque types produce synthetic HIR nodes via query feeding and have no real HIR type to walk.

handle Node::Synthetic by returning an empty type list so the function returns None and the primary WF error is preserved with its original span. this is consistent with how other code handles synthetic nodes (e.g. reachable.rs, hir/mod.rs).

Added two regression tests from the issue.

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