[ORC] Enable use of TargetProcessControl::getMemMgr with ObjectLinkingLayer.

This patch makes ownership of the JITLinkMemoryManager by ObjectLinkingLayer
optional: the layer can still own the memory manager but no longer has to.

Evevntually we want to move to a state where ObjectLinkingLayer never owns its
memory manager. For now allowing optional ownership makes it easier to develop
classes that can dynamically use either RTDyldObjectLinkingLayer, which owns
its memory managers, or ObjectLinkingLayer (e.g. LLJIT).