cmd/protoc-gen-go: change some arrays to slices to save bytes

Using arrays in the generated reflection information adds unnecessary
eq and hash functions being added to the package. Change to slices
to reduce bloat.

Change-Id: I1a4f6d59021644d93dd6c24679b9233141e89a75
Reviewed-on: https://go-review.googlesource.com/c/164640
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>
diff --git a/encoding/testprotos/pb2/test.pb.go b/encoding/testprotos/pb2/test.pb.go
index 8df2ba0..d3a1484 100644
--- a/encoding/testprotos/pb2/test.pb.go
+++ b/encoding/testprotos/pb2/test.pb.go
@@ -1956,8 +1956,8 @@
 
 var File_pb2_test_proto protoreflect.FileDescriptor
 
-var xxx_File_pb2_test_proto_enumTypes [2]protoreflect.EnumType
-var xxx_File_pb2_test_proto_messageTypes [20]protoimpl.MessageType
+var xxx_File_pb2_test_proto_enumTypes = make([]protoreflect.EnumType, 2)
+var xxx_File_pb2_test_proto_messageTypes = make([]protoimpl.MessageType, 20)
 var xxx_File_pb2_test_proto_goTypes = []interface{}{
 	(Enum)(0),                             // 0: pb2.Enum
 	(Enums_NestedEnum)(0),                 // 1: pb2.Enums.NestedEnum
@@ -2068,18 +2068,18 @@
 }
 
 func init() {
-	var messageTypes [20]protoreflect.MessageType
-	var extensionTypes [19]protoreflect.ExtensionType
+	messageTypes := make([]protoreflect.MessageType, 20)
+	extensionTypes := make([]protoreflect.ExtensionType, 19)
 	File_pb2_test_proto = protoimpl.FileBuilder{
 		RawDescriptor:        xxx_File_pb2_test_proto_rawdesc,
 		GoTypes:              xxx_File_pb2_test_proto_goTypes,
 		DependencyIndexes:    xxx_File_pb2_test_proto_depIdxs,
-		EnumOutputTypes:      xxx_File_pb2_test_proto_enumTypes[:],
-		MessageOutputTypes:   messageTypes[:],
-		ExtensionOutputTypes: extensionTypes[:],
+		EnumOutputTypes:      xxx_File_pb2_test_proto_enumTypes,
+		MessageOutputTypes:   messageTypes,
+		ExtensionOutputTypes: extensionTypes,
 	}.Init()
 	messageGoTypes := xxx_File_pb2_test_proto_goTypes[2:][:20]
-	for i, mt := range messageTypes[:] {
+	for i, mt := range messageTypes {
 		xxx_File_pb2_test_proto_messageTypes[i].GoType = reflect.TypeOf(messageGoTypes[i])
 		xxx_File_pb2_test_proto_messageTypes[i].PBType = mt
 	}