Fixed issue with "go to implementation" and symbol overrides

[filipw]

This PR fixes the issue where "find implementation" OmniSharp would not find symbol overrides in child types. We were missing the look up of overrides on the symbols returned from SymbolFinder.FindImplementationsAsync.

Consider the following:

    interface IFoo
    {
        string Hi();
    }

    public class Bar : IFoo
    {
        public virtual string Hi() => "hi";
    }

    public class Baz : Bar
    {
        public override string Hi() => "hello";
    }

In Visual Studio, trying to go to implementation of Hi(), from the interface declaration, would find both Bar.Hi() and Baz.Hi(). However, OmniSharp would find only Bar.Hi().

This is with the fix applied:

[filipw]

@rchande @david-driscoll @DustinCampbell can you guys take a quick look? I'd like to get this in so that it doesn't lay around for ages 🎃

[rchande]

LGTM, thanks. Sorry for the delay.

[filipw]

thanks - there is another PR on the same file(s) so we need to start unclogging them 🙃