no-unused-variable false positive when referenced as a type (rather than a value)

[adidahiya]

code samples below, reported in #191

from @SenHeng:

import Foo = this.that.Foo;

module some.module.blah {
 export  class bar {
    private foo: Foo;
  }
}

Foo is a false positive

---

from @nikklassen:

import DateTimeOpts = Intl.DateTimeFormatOptions;

interface MyDateTimeOpts extends DateTimeOpts {
    timezoneOffset: number;
}

let opts: MyDateTimeOpts;
console.log(opts.timezoneOffset - 1);

DateTimeOpts is a false positive