Work around a circular dependency between IR and MC introduced in r362735

I replaced the circular library dependency with a forward declaration,
but it is only a workaround, not a real fix.

llvm-svn: 362782

llvm/include/llvm/MC/MCSymbolXCOFF.h

@@ -10,10 +10,11 @@
 
 #include "llvm/BinaryFormat/XCOFF.h"
 #include "llvm/MC/MCSymbol.h"
-#include "llvm/IR/GlobalValue.h"
 
 namespace llvm {
 
+class GlobalValue;
+
 class MCSymbolXCOFF : public MCSymbol {
   // The IR symbol this MCSymbolXCOFF is based on. It is set on function
   // entry point symbols when they are the callee operand of a direct call