Namespaces not merged from UMD globals

[RyanCavanaugh]

a.d.ts

export = ns;

export as namespace ns;

declare namespace ns {
  export var x: number;
  export interface IFoo { }
}

b.d.ts

declare namespace ns.something {
  export var p: ns.IFoo;
}

tsc a.d.ts b.d.ts

Expected: No error
Actual: Error that ns has no member IFoo (because the ns UMD global gets shadowed rather than merged)

[RyanCavanaugh]

Talked with @mhegazy and we think this might be the behavior we want anyway -- b.d.ts should convert to UMD or module augmentation

[ghost]

It looks like this was done intentionally in #10354, which makes the export as namespace take no effect if there is already a global (which there is, from b.d.ts). @RyanCavanaugh @mhegazy Can this be closed?