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

[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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@239640 91177308-0d34-0410-b5e6-96231b3b80d8