Fix marshalling declarations of IStream in netstandard reference dll (#631)
Fix marshalling declarations of IStream in netstandard reference dll (#631)

The IStream definition in contract definition of System.Runtime.InteropServices.ComTypes had incorrect marshalling declarations which meant the corert mcg tool couldn't work with it. This fixes it.
ericstj/standard,ericstj/standard,weshaggard/standard,weshaggard/standard