[spec] tsc accepts a TypeAliasDeclaration inside a AmbientExternalModuleDeclaration, the spec doesn't

[sedwards2009]

According to the grammar in the spec a TypeAliasDeclaration not permitted to appear inside a AmbientExternalModuleDeclaration.

For example, on DefinitelyTyped acl.d.ts contains:

declare module "acl" {
  type strings = string|string[];
}

This doesn't conform to the grammar, but tsc 1.5.0-beta is happy to accept it though.

TypeAliasDeclaration just needs to be added to the production of AmbientModuleElement in the spec as far as I can tell.

[ahejlsberg]

Fixed in #4033.