Object.assign return assigned type instead merged

[anion155]

Got 'assing' in my eyes. Found some old bugs created in 16 (#11100). So I made this quick PR.

Situation:

let a = {
  a: 'string',
  b: { c: 4 },
};
let b = {
  a: 5,
  b: { d: 3 },
}
Object.assign({ }, a, b);

Result of this operation is { a: 5, b: { d: 3 } }, but TS says its of { a: string & number; b: { c: number; d: number; } } type, which is obviously wrong (according to mozilla's JS reference).

[sandersn]

We actually decided recently to double down on this incorrect definition of spread and Object.assign in #28234. In practise, the most common use of spread is to spread two identical types. The second most common use is to spread two disjoint types. Spreading two overlapping types is pretty rare, so we decided not to create a type operator for it.

Also note that Assign is still not correct, since it doesn't understand own properties, optional properties, or private properties. getSpreadType handles these aspects, but doesn't work for higher-order cases since we didn't create the spread type operator.