Downlevel for 'let' doesn't account for nested declarations

``` ts
var x;
function foo() {
 let x = 0;
 (function () {
 var _x = 1;
 console.log(x); // Prints 1, should print 0
 })();
}
```

The rename of the 'let' does not account for the _x declared in the nested function. Consequently, it chooses _x as the rename, but now the inner _x shadows it.
