MC: Bit pack MCSymbolData.

On x86_64 this brings it from 80 bytes to 64 bytes. Also make any member
variables private and clean up uses to go through the existing accessors.

NFC.

llvm-svn: 219573
diff --git a/llvm/lib/MC/MachObjectWriter.cpp b/llvm/lib/MC/MachObjectWriter.cpp
index 7a1ab79..7f6d12c 100644
--- a/llvm/lib/MC/MachObjectWriter.cpp
+++ b/llvm/lib/MC/MachObjectWriter.cpp
@@ -41,7 +41,7 @@
 bool MachObjectWriter::
 doesSymbolRequireExternRelocation(const MCSymbolData *SD) {
   // Undefined symbols are always extern.
-  if (SD->Symbol->isUndefined())
+  if (SD->getSymbol().isUndefined())
     return true;
 
   // References to weak definitions require external relocation entries; the