internal/filedesc: remove duplicative Message.unmarshalOptions

A user reported a data race when calling the IsMapEntry and Options accessors
concurrently: golang/protobuf#1701

Originally, I wanted to suggest moving the md.L1.IsMapEntry / md.L1.IsMessageSet
assignments into Message.unmarshalSeedOptions when I realized that these fields
are already set in unmarshalSeedOptions!

Therefore, we can entirely remove Message.unmarshalOptions in favor of
Message.unmarshalSeedOptions. All tests continue to pass, both within Go
Protobuf itself, and also all Google tests (Global Presubmit, cl/832261606).

Resolves golang/protobuf#1701

Change-Id: Ieb4dbddd11f4a123ba01d3c10fed4d16bee0791a
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/720980
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Lasse Folger <lassefolger@google.com>

Author: stapelberg

internal/filedesc/desc_lazy.go

@@ -330,7 +330,6 @@ func (md *Message) unmarshalFull(b []byte, sb *strs.Builder) {
 				md.L1.Extensions.List[extensionIdx].unmarshalFull(v, sb)
 				extensionIdx++
 			case genid.DescriptorProto_Options_field_number:
-				md.unmarshalOptions(v)
 				rawOptions = appendOptions(rawOptions, v)
 			}
 		default:
@@ -356,27 +355,6 @@ func (md *Message) unmarshalFull(b []byte, sb *strs.Builder) {
 	md.L2.Options = md.L0.ParentFile.builder.optionsUnmarshaler(&descopts.Message, rawOptions)
 }
 
-func (md *Message) unmarshalOptions(b []byte) {
-	for len(b) > 0 {
-		num, typ, n := protowire.ConsumeTag(b)
-		b = b[n:]
-		switch typ {
-		case protowire.VarintType:
-			v, m := protowire.ConsumeVarint(b)
-			b = b[m:]
-			switch num {
-			case genid.MessageOptions_MapEntry_field_number:
-				md.L1.IsMapEntry = protowire.DecodeBool(v)
-			case genid.MessageOptions_MessageSetWireFormat_field_number:
-				md.L1.IsMessageSet = protowire.DecodeBool(v)
-			}
-		default:
-			m := protowire.ConsumeFieldValue(num, typ, b)
-			b = b[m:]
-		}
-	}
-}
-
 func unmarshalMessageReservedRange(b []byte) (r [2]protoreflect.FieldNumber) {
 	for len(b) > 0 {
 		num, typ, n := protowire.ConsumeTag(b)