Support string literal completions inside type argument

[mariusschulz]

TypeScript Version: 2.7.0-dev.20180112

Code

interface Foo {
    foo: string;
    bar: string;
}

type SubsetOfFoo = Pick<Foo, "">;

Expected behavior: Completions "foo" and "bar" within "".

Actual behavior: No completions.

Related: #20843