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

The newly created MCDataFragment should inherit Atom (see
MCMachOStreamer::finishImpl). I cannot think of a case to test the
behavior, but this is one step towards folding the Mach-O label
difference below and making Mach-O more similar to ELF.

```
.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  // error: expected relocatable expression // Mach-O
```

Author: MaskRay

llvm/lib/MC/MCSection.cpp

@@ -116,6 +116,8 @@ void MCSection::flushPendingLabels() {
     MCFragment *F = new MCDataFragment();
     getFragmentList().insert(CurInsertionPoint, F);
     F->setParent(this);
+    if (CurInsertionPoint != begin())
+      F->setAtom(std::prev(CurInsertionPoint)->getAtom());
     flushPendingLabels(F, 0, Label.Subsection);
   }
 }