Fix missing 'infer T' in declaration files

[ahejlsberg]

With this PR we properly emit infer T declarations in representations of instantiated conditional types.

Additionally, we report errors on infer T declarations in invalid locations within conditional types nested within other conditional types. Additionally, we now properly locate the owning conditional type for an infer T declaration: It is the innermost containing conditional type in whose extends type the infer T declaration occurs.

Fixes #22755.

[DanielRosenwasser]

Let's make sure this gets into release-2.8 after merging.

[weswigham]

export type BadNested<T> = {x: T extends number ? T : string};

export declare function foo<T>(obj: T): T extends {[K in keyof BadNested<infer P>]: BadNested<infer P>[K]} ? P : never;
export function bar<T>(obj: T) {
    return foo(obj);
}

will still emit invalid declarations for bar (but issue no errors), since we happily inline aliases from time-to-time (even though this breaks conditional types under their current scoping rules). The only real fix I see is to make aliases actually inlinable again by allowing infer types in the other conditional type clauses, but correctly scoping them to the outer scope (rather than them being a deadzone like they are with this PR).

[ahejlsberg]

@weswigham Should be good now.

[weswigham]

👍