[Orc] Rename ObjectLinkingLayer -> RTDyldObjectLinkingLayer.

The current ObjectLinkingLayer (now RTDyldObjectLinkingLayer) links objects
in-process using MCJIT's RuntimeDyld class. In the near future I hope to add new
object linking layers (e.g. a remote linking layer that links objects in the JIT
target process, rather than the client), so I'm renaming this class to be more
descriptive.

llvm-svn: 295636