Fixes #2458.

In ES6 it is illegal to redeclare a lexically bound name (such as the name of a let, const, or class declaration). When we merge internal modules with classes, we emit the following for --target ES6:

// a.ts
class C {
}
module C {
  export var x = 1;
}

// a.js
class C {
}
var C; // error: redeclaration of C
(function (C) {
  C.x = 1;
})(C || (C = {}));

This change elides the var declaration for a module given the following conditions:

• The script target is ES6
• The internal module merges with an existing class declaration in the same lexical scope.

As a result, the new output would be:

class C {
}
(function (C) {
  C.x = 1;
})(C || (C = {}));