internal/impl: add MessageState to every generated message

We define MessageState, which is essentially an atomically set *MessageInfo.
By nesting this as the first field in every generated message, we can
implement the reflective methods on a *MessageState when obtained by
unsafe casting a concrete message pointer as a *MessageState.
The MessageInfo held by MessageState provides additional Go type information
to interpret the memory that comes after the contents of the MessageState.

Since we are nesting a MessageState in every message,
the memory use of every message instance grows by 8B.

On average, the body of ProtoReflect grows from 133B to 202B (+50%).
However, this is offset by XXX_Methods, which is 108B and
will be removed in a future CL. Taking into account the eventual removal
of XXX_Methods, this is a net reduction of 25%.

name          old time/op    new time/op    delta
Name/Value-4    70.3ns ± 2%    17.5ns ± 6%   -75.08%  (p=0.000 n=10+10)
Name/Nil-4      70.6ns ± 3%    33.4ns ± 2%   -52.66%  (p=0.000 n=10+10)

name          old alloc/op   new alloc/op   delta
Name/Value-4     16.0B ± 0%      0.0B       -100.00%  (p=0.000 n=10+10)
Name/Nil-4       16.0B ± 0%      0.0B       -100.00%  (p=0.000 n=10+10)

name          old allocs/op  new allocs/op  delta
Name/Value-4      1.00 ± 0%      0.00       -100.00%  (p=0.000 n=10+10)
Name/Nil-4        1.00 ± 0%      0.00       -100.00%  (p=0.000 n=10+10)

Change-Id: I92bd58dc681c57c92612fd5ba7fc066aea34e95a
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/185460
Reviewed-by: Damien Neil <dneil@google.com>
diff --git a/internal/testprotos/irregular/test.pb.go b/internal/testprotos/irregular/test.pb.go
index 5ea3cb0..6a26b23 100644
--- a/internal/testprotos/irregular/test.pb.go
+++ b/internal/testprotos/irregular/test.pb.go
@@ -18,6 +18,7 @@
 )
 
 type Message struct {
+	state           protoimpl.MessageState
 	OptionalMessage *IrregularMessage            `protobuf:"bytes,1,opt,name=optional_message,json=optionalMessage" json:"optional_message,omitempty"`
 	RepeatedMessage []*IrregularMessage          `protobuf:"bytes,2,rep,name=repeated_message,json=repeatedMessage" json:"repeated_message,omitempty"`
 	RequiredMessage *IrregularMessage            `protobuf:"bytes,3,req,name=required_message,json=requiredMessage" json:"required_message,omitempty"`
@@ -40,10 +41,18 @@
 func (*Message) ProtoMessage() {}
 
 func (x *Message) ProtoReflect() protoreflect.Message {
-	return file_irregular_test_proto_msgTypes[0].MessageOf(x)
+	mi := &file_irregular_test_proto_msgTypes[0]
+	if protoimpl.UnsafeEnabled && x != nil {
+		ms := protoimpl.X.MessageStateOf(protoimpl.Pointer(x))
+		if ms.LoadMessageInfo() == nil {
+			ms.StoreMessageInfo(mi)
+		}
+		return ms
+	}
+	return mi.MessageOf(x)
 }
 
-func (m *Message) XXX_Methods() *protoiface.Methods {
+func (x *Message) XXX_Methods() *protoiface.Methods {
 	return file_irregular_test_proto_msgTypes[0].Methods()
 }
 
@@ -193,9 +202,11 @@
 	if !protoimpl.UnsafeEnabled {
 		file_irregular_test_proto_msgTypes[0].Exporter = func(v interface{}, i int) interface{} {
 			switch v := v.(*Message); i {
-			case 5:
-				return &v.sizeCache
+			case 0:
+				return &v.state
 			case 6:
+				return &v.sizeCache
+			case 7:
 				return &v.unknownFields
 			default:
 				return nil