Lint x != a || x != b

I recently came across a bug in production code of the form:

if x != a || x != b {
    // create entry with value x
}

This was a bug, but it's not terribly obvious in that form. My guess is that it was copy-pasted from a different part of the code that did a something like:

if x == a || x == b {
    // use x
}

which was then inverted but without changing the || to &&. If we use De Morgan's law, it becomes clearer why a test like x != a || x != b is almost certainly wrong:

x != a || x != b

is equivalent to

!(x == a && x == b)

which makes it easier to see that the inner expression is false unless a == b. But this way of writing it is very unclear. If that's the actual intent, this reformulation is more legible (assuming transitivity of std::cmpEq:

a == b && x == a

Since I think x != a || x != b is more likely the result of a a mistaken conversion of x == a || x == b, it's worth linting.

Start from the Rust condition examples in the issue and define the exact pattern the lint should detect, including the intended comparison semantics. Identify the relevant Clippy lint entry point and add coverage showing that the mistaken || form is reported while the legible or intended alternatives are not; done means the lint handles the motivating case without inappropriate warnings.