Support completion details for string literal completions

[ghost]

TypeScript Version: 2.7.0-dev.20180123

Search Terms: string completion details

Code

/// <reference path="fourslash.ts" />

////const x: "a" = "/**/";

goTo.marker();
verify.completionListContains("a", "text", "doc", "kind", undefined, undefined);

Expected behavior:

Successfully get completion details.

Actual behavior:

NPE because getCompletionEntryDetails returns undefined.