[MC] flushPendingLabels: set Atom for new fragment after D71368

Fixes: c26c5e47 (essentially a no-op)

The newly created MCDataFragment should inherit Atom (see
MCMachOStreamer::finishImpl). To the best of my knowledge, this change cannot be
tested at present, but this is important to ensure
MCExpr.cpp:AttemptToFoldSymbolOffsetDifference gives the same result in case we
evaluate the expression again with a MCAsmLayout.

In the following case,
```
.section __DATA,xray_instr_map
lxray_sleds_start1:
.space 16
Lxray_sleds_end1:
.section __DATA,xray_fn_idx
.quad (Lxray_sleds_end1-lxray_sleds_start1)>>4 // can be folded without a MCAsmLayout
```

When we have a MCAsmLayout, without this change, evaluating
(Lxray_sleds_end1-lxray_sleds_start1)>>4 again will fail due to
`FA->getAtom() == nullptr && FB.getAtom() != nullptr` in
MachObjectWriter::isSymbolRefDifferenceFullyResolvedImpl, called by
AttemptToFoldSymbolOffsetDifference.