[7392] [cpp] Remove dead code path for map key of type enum in JSON parsing (#16567)

[copybara-service]

[7392] [cpp] Remove dead code path for map key of type enum in JSON parsing (#16567)

Changes

Remove dead code path -- we don't allow enums to be map keys (proto2 spec, proto3 spec). In other words the case block case FieldDescriptor::TYPE_ENUM is dead code. Potential enum type keys will be caught in default: return lex.Invalid("unsupported map key type"); block below similar to other unsupported map key types like double.

Motivation

While working on fixing IgnoreUnknownEnumStringValueInMap conformance tests for cpp (related issue) I stumbled upon a bug where we pass the wrong field parameter to the enum parsing function.

In this scope:

• the variable field is a map field of the message that holds the map. This field is not of enum type, it's a repeated message of map entires.
• the variable key_field is the key field of the map message entry. This field is the enum type that we need to parse here.

The function is long, so I clarified it here:

template <typename Traits>
absl::Status ParseMap(JsonLexer& lex, Field<Traits> field, Msg<Traits>& msg) {
  (..)
  return lex.VisitObject(
      [&](LocationWith<MaybeOwnedString>& key) -> absl::Status {
          (..)
          return Traits::NewMsg(
            field, msg,
            [&](const Desc<Traits>& type, Msg<Traits>& entry) -> absl::Status {
              auto key_field = Traits::KeyField(type);
              switch (Traits::FieldType(key_field)) {
                (..)
                case FieldDescriptor::TYPE_ENUM: {
                  MaybeOwnedString key_str = key.value;
                  auto e = ParseEnumFromStr<Traits>(lex, key_str, /** bug here **/ field);

The correct reference should be key_field.

Instead of fixing the bug and leaving the dead code, it's better to remove the dead block alltogether.

Closes #16567

COPYBARA_INTEGRATE_REVIEW=#16567 from noom:anton--7392--fix-map-key-nit d992b8a
FUTURE_COPYBARA_INTEGRATE_REVIEW=#16567 from noom:anton--7392--fix-map-key-nit d992b8a