Expose the ELFObjectFile class directly in the Object/ELF.h header, similarly

to what's done for MachO and COFF. This allows advanced uses of the class to
be implemented outside the Object library. In particular, the DyldELFObject
subclass is now moved into its logical home - ExecutionEngine/RuntimeDyld.

This patch was reviewed by Michael Spencer.

llvm-svn: 150327