[ORC][JITLink] Fix MachO absolute symbol handling, add test case.

This patch removes the unintended resolution of locally scoped absolute symbols
(which was causing unexpected definition errors).

It stops using the JITSymbolFlags::Absolute flag (it isn't set or used elsewhere,
and causes mismatch-flags asserts), and adds JITSymbolFlags::Exported to default
scoped absolute symbols.

Finally, we now set the scope of absolute symbols correctly in
MachOLinkGraphBuilder.