Do not emit extra var decls for merged enums/namespaces

[rbuckton]

This change removes the unnecessary extra var declarations when we merge enums or namespaces with preceding declarations. This also helps to avoid the extra export var declarations in #11258.

Given the following source:

// @module: es6
// @target: es5
export class C { }
export namespace C {
  export const x = 1;
}

enum E { x = 0 }
enum E { y = 1 }

Previously we would emit:

export var C = (function () { 
  function C() {
  }
  return C;
})();
export var C;
(function (C) {
  C.x = 1;
})(C || (C = {}));
var E;
(function (E) {
  E[E["x"] = 0] = "x";
})(E || (E = {}));
var E;
(function (E) {
  E[E["y"] = 1] = "y";
})(E || (E = {}));

With this change we now emit:

export var C = (function () { 
  function C() {
  }
  return C;
})();
(function (C) {
  C.x = 1;
})(C || (C = {}));
var E;
(function (E) {
  E[E["x"] = 0] = "x";
})(E || (E = {}));
(function (E) {
  E[E["y"] = 1] = "y";
})(E || (E = {}));

Fixes #11258

[mhegazy]

Fixes #5553

[mihailik]

Does it 'merges' var declarations in case of outFile emit?

[rbuckton]

No, it will not remove var declarations for namespaces or enums merged across files when emitting to a single file, just from within a single source file.