Disallow merging for default export declarations

[DanielRosenwasser]

Fixes #3095.

With this change, the following code will now cause an error:

export default function Foo() {
}

export namespace Foo {
    export var x = 10;
}

The idea is that the namespace cannot overlap in the same space with the function because the two do not agree in their modifiers.

However, the following will continue to work

export default function Foo() {
}

export interface Foo {
    export var x = 10;
}

This will give the module a function for a default export, and an exported interface Foo. The two will not be merged.

[JsonFreeman]

Should end with the word "spaces" not "flags".

[JsonFreeman]

👍