Remove unnecessary includes (NFC)

(cherry picked from commit 8094600)

Author: adrian-prantl

lldb/source/Plugins/Language/Swift/SwiftArray.cpp

@@ -23,8 +23,6 @@
 // FIXME: we should not need this
 #include "Plugins/Language/ObjC/Cocoa.h"
 
-#include "llvm/ADT/StringRef.h"
-
 using namespace lldb;
 using namespace lldb_private;
 using namespace lldb_private::formatters;