Wrong "This declaration is never used." warning when variables are used only in reassignment LHS in index expressions

[Igor Rončević]

E.g., this example will emit a false warning:

script;

fn main() {
    let i = 0;
    //  - This declaration is never used.  <<<----- WRONG WARNING.
    let mut array = [1, 2, 3];

    array[i] = 0; // <<<----- `i` is used here.
}