Derive tuple element's type accessed by const enum index

[lazdmx]

It would be handy if TS will use values of const enum literals when deriving element type.

type T = [number, string]
const enum Idx {
    Num, Str
}
var t: T = [0, '1']
var x = t.shift()
var y = t[Idx.Num] // expect number type but got string | number
var z = t[Idx.Str] // expect string type but got string | number

[RyanCavanaugh]

Dupe #3411