Add LLVM_GSL_POINTER to llvm::function_ref.

Author: hokein

llvm/include/llvm/ADT/STLFunctionalExtras.h

@@ -37,7 +37,7 @@ namespace llvm {
 template<typename Fn> class function_ref;
 
 template<typename Ret, typename ...Params>
-class function_ref<Ret(Params...)> {
+class LLVM_GSL_POINTER function_ref<Ret(Params...)> {
   Ret (*callback)(intptr_t callable, Params ...params) = nullptr;
   intptr_t callable;