MC: Change MCFragment::Atom to an MCSymbol, NFC

Change `MCFragment::Atom` from an `MCSymbolData` to an `MCSymbol`,
moving in the direction of removing the back-pointer.

llvm-svn: 237497
diff --git a/llvm/lib/MC/MCMachOStreamer.cpp b/llvm/lib/MC/MCMachOStreamer.cpp
index ee3f358..b381e578 100644
--- a/llvm/lib/MC/MCMachOStreamer.cpp
+++ b/llvm/lib/MC/MCMachOStreamer.cpp
@@ -478,11 +478,11 @@
   // symbol.
   for (MCAssembler::iterator it = getAssembler().begin(),
          ie = getAssembler().end(); it != ie; ++it) {
-    MCSymbolData *CurrentAtom = nullptr;
+    const MCSymbol *CurrentAtom = nullptr;
     for (MCSectionData::iterator it2 = it->begin(),
            ie2 = it->end(); it2 != ie2; ++it2) {
       if (MCSymbolData *SD = DefiningSymbolMap.lookup(it2))
-        CurrentAtom = SD;
+        CurrentAtom = &SD->getSymbol();
       it2->setAtom(CurrentAtom);
     }
   }