The call_func_with_field! macro uses dead code willingly to infer
the appropriate type. This has started adding a new warning:
error: unused variable: `value__`
79 | break phantom__(&{ let value__: $typ; value__.$($field).+ })
So shut it up together with the existing unreachable_code warning.
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>