Fixes #606

This supports completions in string literals in the following locations:

1. argument to a call expression whose target parameter has a StringLiteralType

   declare function f(a: "A", b: number): void;
   declare function f(a: "B", b: number): void;
   declare function f(a: "C", b: number): void;
   
   f(" // A | B | C

2. argument expression of an element access expression

   var o = {
   "#invlaid identifier name": 1
   };
   
   o["| // #invlaid identifier name

3. a string literal contextually typed by a string literal type

   type Options = "Option 1" | "Option 2" | "Option 3";
   var x: Options = "| // Option 1 | Option 2 | Option 3

This PR does not cover:

• module name completion
• changing o. to o[""] if the completion entry is not a valid identifier name