XCOFF.h - replace StringRef.h include with forward declaration. NFC.

Move StringRef.h include to XCOFF.cpp

llvm/include/llvm/BinaryFormat/XCOFF.h

@@ -13,10 +13,11 @@
 #ifndef LLVM_BINARYFORMAT_XCOFF_H
 #define LLVM_BINARYFORMAT_XCOFF_H
 
-#include "llvm/ADT/StringRef.h"
 #include <cstdint>
 
 namespace llvm {
+class StringRef;
+
 namespace XCOFF {
 
 // Constants used in the XCOFF definition.

llvm/lib/BinaryFormat/XCOFF.cpp

@@ -7,6 +7,7 @@
 //===----------------------------------------------------------------------===//
 
 #include "llvm/BinaryFormat/XCOFF.h"
+#include "llvm/ADT/StringRef.h"
 
 using namespace llvm;