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/MCAssembler.cpp b/llvm/lib/MC/MCAssembler.cpp
index ff230dfd..197b922 100644
--- a/llvm/lib/MC/MCAssembler.cpp
+++ b/llvm/lib/MC/MCAssembler.cpp
@@ -460,10 +460,10 @@
   return false;
 }
 
-const MCSymbolData *MCAssembler::getAtom(const MCSymbolData *SD) const {
+const MCSymbol *MCAssembler::getAtom(const MCSymbolData *SD) const {
   // Linker visible symbols define atoms.
   if (isSymbolLinkerVisible(SD->getSymbol()))
-    return SD;
+    return &SD->getSymbol();
 
   // Absolute and undefined symbols have no defining atom.
   if (!SD->getFragment())