RepeatedPtrIterator is a view type

After llvm/llvm-project#177660, Clang tracks iterator lifetimes through transparent functions (like .begin(), operator*) and allows to detect the following case:
```cpp
const UafSubProto* ReturnSubProtoFromForLoop(bool condition) {
  const UafSubProto* p = nullptr;
  UafProto proto;
  for (const UafSubProto& sub_proto : proto.repeated_sub_protos()) {  // expected-error {{address of stack memory is returned later}}
    if (condition) {
      return &sub_proto;  // expected-note {{returned here}}
    }
  }
  for (const UafSubProto& sub_proto : proto.repeated_sub_protos()) {  // expected-error {{address of stack memory is returned later}}
    if (condition) {
      p = &sub_proto;
      return p;  // expected-note {{returned here}}
    }
  }
  return p;
}
```

PiperOrigin-RevId: 865027461

Author: protobuf-github-bot

src/google/protobuf/repeated_ptr_field.h

@@ -1980,7 +1980,7 @@ class RustRepeatedMessageHelper {
 // This code based on net/proto/proto-array-internal.h by Jeffrey Yasskin
 // (jyasskin@google.com).
 template <typename Element>
-class RepeatedPtrIterator {
+class ABSL_ATTRIBUTE_VIEW RepeatedPtrIterator {
  public:
   using iterator = RepeatedPtrIterator<Element>;
   using iterator_category = std::random_access_iterator_tag;