Fix crash when getting member completion for an object literal

The following code caused a crash:
```TS
function f(a: { xa?: number; xb?: number; }) { }
var xc;
f({
    $
```
This changes the find the relevant token at the beginning of getCompletionsAtPosition. This fixes the crash, but the following case still does not work:

```TS
function f(a: { xa: number; xb: number; }) { }
var xc;
f({
    $
```
This represents the next item to be done, which is making the type checker give us the right contextual type.
Is there a particular question you want answered? Basically, we start with a Fidelity node, and we have to find the corresponding AST node. @mhegazy said he is working on removing the Fidelity part, so that might make it clearer.