Improve inference to union and intersection types

[ahejlsberg]

This PR improves inference to union and intersection types containing multiple naked type variables. Previously we would make no inferences to such union and intersection types (for unrelated historical reasons that no longer apply). We now make the the proper inferences:

declare function f1<T, U>(x: T | U): T | U;
declare function f2<T, U>(x: T & U): T & U;

let x1: string = f1('a');
let x2: string = f2('a');

Previously both assignments were errors because we'd infer {} for T and U.

Fixes #29815.

[weswigham]

Since I'm curious, what prompted us to only infer to one naked type parameter originally, anyway?

[ahejlsberg]

@weswigham I think it was a holdover from when we didn't have priorities associated with inference candidates.

[weswigham]

Huh. I wonder why we didn't remove it when we added the NakedTypeParameter priority, then.
🤷