[Emit] Module name not underscored when conflicts with parameter name

Input:

``` TypeScript
module Y {    function f(Y) { return g; }    export var g;}
```

Emit:

``` TypeScript
var Y;
(function (Y) {
    function f(Y) {
        return Y.g;   <-- wrong
    }
    Y.g;})(Y || (Y = {}));
```

Expected emit:

``` TypeScript
var Y;
(function (_Y) {
    function f(Y) {
        return _Y.g;
    }
    _Y.g;})(Y || (Y = {}));
```
