With this PR we fix the following long standing issue:

declare function obj(x: object): void;

function foo(x: unknown) {
    if (typeof x === 'object' && x !== null) obj(x);  // Ok
    if (x !== null && typeof x === 'object') obj(x);  // Was error, now ok
}

Above, the order of the checks mattered because unknown is an "indivisible" non-union type and therefore a check for x !== null couldn't narrow x in the second if statement. We implemented a partial fix in #37507, but this only worked for truthy checks combined with typeof checks using the && operator in the same expression.

With this PR we remove the partial fix and instead introduce an internal nonNullUnknownType that is used in control flow analysis to represent an unknown type that is known to be non-null. The non-null unknown type results when the unknown type is subjected to a truthiness check or an x !== null check, and when the non-null unknown is subsequently subjected to a typeof x === 'object' check, we narrow to object instead of object | null. Because we rely on control flow analysis, this solution properly reflects the effects of non-null checks in separate expressions or statements:

function bar(x: unknown) {
    if (!x) {
        return;
    }
    if (typeof x === 'object') {
        obj(x);  // Was error, now ok
    }
}

Fixes #28131.