Ignore underscore-prefixed args for needless_pass_by_value lint
When a user explicitly tags a param as unused (yet?), there is no need to raise another lint on it.
fixes#7295
Note that I had to rename all `_*` params in the tests, but kept the variable name length to avoid extra changes in the expected output.
changelog: [`needless_pass_by_value`]: do not warn if the argument name starts with an `_`
083e20a6dc