immutable loop condition lint gives up on mutated upvars

Currently the immutable loop condition lint gives up on code like the following, where the variable being mutated is an upvar:

let (mut a, mut b) = (0, 0);

// this should be okay
// (and it is, but only because clippy gives up; it doesn't
//  know how to compare the 'a' in the body to the 'a' in the condition)
let mut f = || {
    while a < 5 {
        a += 1;
    }
};
f();

// this should produce an error (but clippy doesn't even try)
let mut f = || {
    while a < 5 {
        b += 1;
    }
};
f();

This is a known problem with a FIXME in the source, but I think that fixing it (or at least, discovering how to fix it) will be a useful step towards implementing #2914, which requires much more reliable ways of discovering whether a given variable is used.

Start by tracing the immutable loop condition lint and its FIXME around comparing variables captured as closure upvars. Reproduce both Rust closure examples, then verify that mutating b while testing a produces a diagnostic while mutating a remains accepted; consider how this supports issue #2914.