The composites x/tools/analysis pass errors on this use of
reflect.ValueError with:
```
internal/impl/message_reflect_field.go:541:10: reflect.ValueError struct literal uses unkeyed fields (composites)
```
This patch adds the missing field names.
Change-Id: I1a175aad6e9724c27f2469adc1f3a9b1e8c61e85
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/553915
Reviewed-by: Christian Höppner <hoeppi@google.com>
Reviewed-by: Michael Stapelberg <stapelberg@google.com>
Auto-Submit: Michael Stapelberg <stapelberg@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>