[JITLink] Fix access of moved-from argument in 265d6bda.

The LinkGraph that is passed into the ELFJITLinker_x86_64 constructor is handed
off to the JITLinker base class, so can't be used in the constructor body. This
commit adds a getGraph method to JITLinker, and updates ELFJITLinker_x86_64 to
use that.