Introduces a new layer interface, LinkGraphLayer, that can be used to
add LinkGraphs to an ExecutionSession.
This patch moves most of ObjectLinkingLayer's functionality into a new
LinkGraphLinkingLayer which should (in the future) be able to be used
without linking libObject. ObjectLinkingLayer now inherits from
LinkGraphLinkingLayer and just handles conversion of object files to
LinkGraphs, which are then handed down to LinkGraphLinkingLayer to be
linked.
