Allow captured let/const in loops

Previously an error, now supported in TypeScript 1.8.let/const declarations within loops and captured in functions are now emitted to correctly match let/const freshness semantics.

Example

let list = [];
for (let i = 0; i < 5; i++) {
    list.push(() => i);
}
list.forEach(f => console.log(f()));

is compiled to:

var list = [];
var _loop_1 = function(i) {
    list.push(function () { return i; });
};
for (var i = 0; i < 5; i++) {
    _loop_1(i);
}
list.forEach(function (f) { return console.log(f()); });

And results in

0
1
2
3
4