Provide ostream output for StringRef.

include/llvm/StringRef.h

@@ -14,6 +14,7 @@
 #include <cassert>
 #include <cstring>
 #include <limits>
+#include <ostream>
 #include <string>
 #include <utility>
 
@@ -523,6 +524,11 @@ namespace llvm {
     return buffer.append(string.data(), string.size());
   }
 
+  inline std::ostream &operator<<(std::ostream &os, StringRef string) {
+    os.write(string.data(), string.size());
+    return os;
+  }
+
   /// @}
 
   // StringRefs can be treated like a POD type.