New Lint: Don't modify loop condition variables more than once per iteration

Based on this HIC++ guideline. To give you a short example of what would be linted:

while do_it != DoNothing {
    do_it = do_wörk();
    if special_case { do_it = magic(); } // WARNING: dude, you already modified `do_it`!
}

The idea is that for all mutable variables that are checked in a while loop's condition, this variable should only be modified once per iteration to simplify reasoning about the loop's control flow.

Start with the issue's HIC++ guideline and the Rust example to define what counts as a loop-condition variable being modified more than once. Locate the Clippy entry point for implementing new lints and add coverage for the shown while-loop behavior; done means the repeated modification is reported without incorrectly flagging valid loops.