"Unassigned parameter" analysis doesn't account for local var declarations

In JS, it's legal to have a var declaration merge with a parameter declaration. By doing this and creating one with an initializer, you can get around TypeScript 4.4's "is this parameter ever assigned to?" checks for aliased conditions.

function foo(x: string | number) {
    // Try uncommenting (1) and (2) 

    // (1)
    // var x: string | number = 42;

    // (2)
    // x = 42;

    const a = typeof x === "string";
    if (a) {
        x // observe the type switch between `string` and `never` here.
    }
}

Start with the issue's foo example and compare the commented var x declaration with the direct assignment case. Trace the analysis that decides whether a parameter is ever assigned, then verify that a local var declaration with an initializer is accounted for without producing the incorrect narrowing shown in the example.