yaml2obj.h - cleanup includes and forward declaration. NFC.

Reduce StringRef.h/Error.h includes to just the necessary STLExtras.h include and StringRef/Twine forward declarations
Remove unused Expected<> forward declaration

Author: RKSimon

llvm/include/llvm/ObjectYAML/yaml2obj.h

@@ -11,14 +11,14 @@
 #ifndef LLVM_TOOLS_YAML2OBJ_YAML2OBJ_H
 #define LLVM_TOOLS_YAML2OBJ_YAML2OBJ_H
 
-#include "llvm/ADT/StringRef.h"
-#include "llvm/Support/Error.h"
+#include "llvm/ADT/STLExtras.h"
 #include <memory>
 
 namespace llvm {
 class raw_ostream;
 template <typename T> class SmallVectorImpl;
-template <typename T> class Expected;
+class StringRef;
+class Twine;
 
 namespace object {
 class ObjectFile;