Either type not expanded for overload resolution

[mjbvz]

From microsoft/vscode#28293

TypeScript Version: 2.4.0-20170609
Code

interface IFace {
	do(x: number): number
	do(x: string): string
}

function func(param: number | string, obj: IFace) {
	obj.do(param);
}

Expected behavior:
No compile errors. We have do overloads for both both types of params

Actual behavior:
Compile error. No overload found for do with argument of type number | string

// cc @roblourens

[mhegazy]

duplicate of #14107