MC: Change MCAssembler::Symbols to store MCSymbol, NFC

Instead of storing a list of the `MCSymbolData` in use, store the
`MCSymbol`s.  Churning in the direction of removing the back pointer
from `MCSymbolData`.

llvm-svn: 237496
diff --git a/llvm/lib/MC/WinCOFFObjectWriter.cpp b/llvm/lib/MC/WinCOFFObjectWriter.cpp
index 11a8379..261534e 100644
--- a/llvm/lib/MC/WinCOFFObjectWriter.cpp
+++ b/llvm/lib/MC/WinCOFFObjectWriter.cpp
@@ -642,9 +642,9 @@
   for (const auto &Section : Asm)
     DefineSection(Section);
 
-  for (MCSymbolData &SD : Asm.symbols())
-    if (ExportSymbol(SD.getSymbol(), Asm))
-      DefineSymbol(SD, Asm, Layout);
+  for (const MCSymbol &Symbol : Asm.symbols())
+    if (ExportSymbol(Symbol, Asm))
+      DefineSymbol(Symbol.getData(), Asm, Layout);
 }
 
 bool WinCOFFObjectWriter::IsSymbolRefDifferenceFullyResolvedImpl(