Fixes #16457 (first half). This allows the following example to get the correct contextual type of Y for the object literal with type: 'y'.

interface X {
    type: 'x';
    value: 1 | 2 | 3;
    xtra: number;
}

interface Y {
    type: 'y';
    value: 11 | 12 | 13;
    ytra: number;
}

let xy: X | Y = {
    type: 'y',
    value: 11,
    ytra: 12
};

Note that many properties look like discriminants, but aren't particularly good ones. For example, the original example from #16457 has X.value : string and Y.value: 'none' | 'some'. string | 'none' | 'some' is not a good discriminant and is not helped by this PR.