Mechanism for getting from metadata symbol to underlying handle or raw metadata token

It would be useful to be able to correlate a Roslyn symbol originating in metadata to PDBs, IL, and lower level metadata.

I've experimented with it and I'll submit the following two alternative proposals

1. Expose the handle via a new interface that can be queried

https://github.com/dotnet/roslyn/compare/master...nguerrera:metadata-symbol

public interface IMetadataSymbol
{
    System.Reflection.Metadata.Handle MetadataHandle { get; }
}

internal class PENamedTypeSymbol : NamedTypeSymbol, IMetadataSymbol
{
   public System.Reflection.Metadata.Handle MetadataHandle => _handle;
    ..
}

// etc. for all of the PEXxxSymbols for C# and VB.

2. Expose the handle directly from ISymbol

https://github.com/dotnet/roslyn/compare/master...nguerrera:metadata-symbol-2

public interface ISymbol
{
    System.Reflection.Metadata.Handle MetadataHandle { get; }
    ...
}

internal class Symbol : ISymbol
{
     public virtual System.Reflection.Metadata.Handle MetadataHandle => default(Handle);
     ...
}

internal class PENamedTypeSymbol : NamedTypeSymbol
{
    public override System.Reflection.Metadata.Handle MetadataHandle => _handle;
    ...
}

// etc. for all of the PEXxxSymbols for C# and VB.

So non-metadata symbols simply return default(Handle) which has IsNil property equal to true.

I started with (1) under the assumption that we could not modify an existing interface, but I have since learned about Roslyn's "internal-implementation-only" policy for symbols. I now think (2) is much better and cleaner and aligns with the recent addition of Metadata properties on IAssemblySymbol and IModuleSymbol.