[ORC] Update JITCompileCallbackManager to support multi-threaded code.

Previously JITCompileCallbackManager only supported single threaded code. This
patch embeds a VSO (see include/llvm/ExecutionEngine/Orc/Core.h) in the callback
manager. The VSO ensures that the compile callback is only executed once and that
the resulting address cached for use by subsequent re-entries.

llvm-svn: 333490