Partial rewrite of `unused_io_account` to lint over Ok(_).
Moved the check to `check_block` to simplify context checking for
expressions and allow us to check only some expressions.
For match (expr, arms) we emit a lint for io ops used on `expr` when an
arm is `Ok(_)`. Also considers the cases when there are guards in the
arms. It also captures `if let Ok(_) = ...` cases.
For `Ok(_)` it emits a note indicating where the value is ignored.
changelog: False Negatives [`unused_io_amount`]: Extended
`unused_io_amount` to catch `Ok(_)`s in `If let` and match exprs.
f73879e096