Better error messages when outer 'this' has the right type

[DanielRosenwasser]

From @Andy-MS at #26447

we should also have a good error message in the following situation:

class C {
    m() {
        this.m();
        function f() {
            this.m();
        }
    }
}

This is a common error and it's not obvious why this works in one place but not the other. The error message should indicate the place where this was shadowed.