[ORC] Add ErrorSuccess and void specializations to AsyncHandlerTraits.

This will allow async handlers to be added that return void or Error::success().
Such handlers are expected to be common, since one of the primary uses of
addAsyncHandler is to run the body of the handler in a detached thread, in which
case the main handler returns immediately and does not need to provide an Error
value.

llvm-svn: 312746