add regression tests for the fn sig ice

[im-lunex]

follow-up to #160628 which fixed the suggest_add_reference_to_arg fn_sig ice

adds the regression tests @teor2345 asked for in review

• closure call with a missing-field struct literal (iced without the guard)
• method call whose where-clause fails (covers the method-call branch)

the impl def_id case cant be constructed, those obligations get dropped during probing and turn into e0599 instead

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