Unused assignments to an unused variable should trigger only the
`unused_variables` lint and not also the `unused_assignments` lint.
This was previously implemented by checking whether the span of the
assignee was within the span of the binding pattern, however that failed
to capture situations was imported from elsewhere (eg from the input
tokenstream of a proc-macro that generates the binding pattern).
By comparing the span of the assignee to those of the variable
introductions instead, a reported stable-to-stable regression is
resolved.
This fix also impacted some other preexisting tests, which had
(undesirably) been triggering both the `unused_variables` and
`unused_assignments` lints on the same initializing assignment; those
tests have therefore now been updated to expect only the former lint.
22b3f59882