GC bridge: Include non-bridged objects in the exported object graph

GC bridge: Include non-bridged objects in the exported object graph

Introduces a mechanism for non-bridged SCCs which lay between bridged
SCCs in the SCC forest to be reported to the bridge client. In this
version, such non-bridged SCCs are always reported. The result is that
most GCs get a little slower but GCs in which the "double fan" shape
appears (many bridged objects link to one C# object which links to
many bridged objects) become massively faster.

Because before clients were allowed to assume exported SCCs were
always bridged, SGEN_BRIDGE_VERSION has been incremented.