[Orc] Fix a bug in the CompileOnDemand layer where stub decls were not cloned

into partitions. Also, add an option to clone stub definitions (not just decls)
into partitions: these definitions could be inlined in some places to avoid the
overhead of calling via the stub.

Found by inspection - no test case yet, although I plan to add a unit test for
this once the CompileOnDemand layer refactoring settles down.

llvm-svn: 239640