Improve contextual typing by generic rest parameters

[ahejlsberg]

This PR improves contextual typing by generic rest parameters such that we can infer a tuple type from the function expression or arrow function being contextually typed while simultaneously assigning contextual parameter types to the function expression or arrow function. Consider:

declare function pipe<A extends any[], B, C>(f: (...args: A) => B, g: (x: B) => C): (...args: A) => C;

let x1 = pipe(() => true, b => !b);  // () => boolean
let x2 = pipe(x => "hello", s => s.length);  // (x: any) => number
let x3 = pipe((x, y) => ({ x, y }), o => o.x === o.y);  // (x: any, y: any) => boolean
let x4 = pipe((x: number) => x * x, x => "" + x);  // (x: number) => string

In the examples above, the type inferred for A is [], [any], [any, any], and [number] respectively. Previously, in all but the last example we'd infer any[] for A.

[weswigham]

The cloneTypeMapper call used to create the returnType mapper inside call resolution doesn't pass InferenceFlags.NoDefault with this change. Intentional?

[ahejlsberg]

Yes, it's intentional. We only want the InferenceFlags.NoDefault behavior here (because we're transposing inferences from the outer call mapper and don't want an absence of inferences in the outer mapper to turn into what appears to be a concrete inference of the default).