MC: Make MCSymbolData::Symbol private

Make the back-pointer from `MCSymbolData` to `MCSymbol` private,
preparing to remove the back pointer entirely.  I've already updated all
the users, although for now it's still used to indicate whether
`MCSymbol::Data` has been initialized.

llvm-svn: 237868

llvm/include/llvm/MC/MCSymbol.h

@@ -71,8 +71,6 @@ public:
   /// @{
   bool isInitialized() const { return Symbol; }
 
-  const MCSymbol &getSymbol() const { return *Symbol; }
-
   MCFragment *getFragment() const { return Fragment.getPointer(); }
   void setFragment(MCFragment *Value) { Fragment.setPointer(Value); }