Allow enum types to be index parameters in array interfaces

[iravanchi]

We can do this:

memberName: { [index: number]: string };

And then use enums as index signature parameters, since they are some kind of numbers:

this.memberName[EnumType.EnumMember] = "";

But the language does not allow defining the index signature parameters to be an enum type, like this:

memberName: { [index: EnumType]: string };

Which can be used the same way, and the compiler can check that the references are all using the EnumType, not an arbitrary number.

[danquirk]

See #2491