Teach Descriptors about proto3 optional.

- Add methods to FieldDescritor to check attributes of the field (modeling the
  C++ changes).
- Add a method to OneofDescriptor to check if it is synthetic (i.e. made for
  proto3 optional support).
- Add methods to Descriptor to get the non synthetic oneofs.
- Add a new test file to unittest the apis.
- Add --experimental_allow_proto3_optional to protoc invoke in the Makefile.

Author: thomasvl

Makefile

@@ -53,7 +53,8 @@ GOOGLE_PROTOBUF_CHECKOUT?=../protobuf
 # previously installed one), we use a custom output name (-tfiws_out).
 PROTOC_GEN_SWIFT=.build/debug/protoc-gen-swift
 GENERATE_SRCS_BASE=${PROTOC} --plugin=protoc-gen-tfiws=${PROTOC_GEN_SWIFT}
-GENERATE_SRCS=${GENERATE_SRCS_BASE} -I Protos
+# Until the flag isn't needed, add the flag to enable proto3 optional.
+GENERATE_SRCS=${GENERATE_SRCS_BASE} -I Protos --experimental_allow_proto3_optional
 
 # Where to find the Swift conformance test runner executable.
 SWIFT_CONFORMANCE_PLUGIN=.build/debug/Conformance
@@ -167,6 +168,7 @@ CONFORMANCE_PROTOS= \
 
 SWIFT_DESCRIPTOR_TEST_PROTOS= \
 	Protos/pluginlib_descriptor_test.proto \
+	Protos/pluginlib_descriptor_test2.proto \
 	${PLUGIN_PROTOS}
 
 XCODEBUILD_EXTRAS =
@@ -388,7 +390,9 @@ regenerate-test-protos: build ${PROTOC_GEN_SWIFT} Protos/generated_swift_names_e
 		${TEST_PROTOS}
 
 Tests/SwiftProtobufPluginLibraryTests/DescriptorTestData.swift: build ${PROTOC_GEN_SWIFT} ${SWIFT_DESCRIPTOR_TEST_PROTOS}
+	# Until the flag isn't needed, add the flag to enable proto3 optional.
 	@${PROTOC} \
+		--experimental_allow_proto3_optional \
 		--include_imports \
 		--descriptor_set_out=DescriptorTestData.bin \
 		-I Protos \

Protos/pluginlib_descriptor_test.proto

@@ -80,3 +80,29 @@ service SomeService {
   rpc Foo(google.protobuf.DescriptorProto) returns (google.protobuf.compiler.Version);
   rpc Bar(TopLevelMessage) returns (TopLevelMessage2);
 }
+
+message Proto2MessageForPresence {
+
+  required string req_str_field = 1;
+  required int32 req_int32_field = 2;
+  required TopLevelEnum req_enum_field = 3;
+  required TopLevelMessage req_message_field = 4;
+
+  optional string opt_str_field = 11;
+  optional int32 opt_int32_field = 12;
+  optional TopLevelEnum opt_enum_field = 13;
+  optional TopLevelMessage opt_message_field = 14;
+
+  repeated string repeat_str_field = 21;
+  repeated int32 repeat_int32_field = 22;
+  repeated TopLevelEnum repeat_enum_field = 23;
+  repeated TopLevelMessage repeat_message_field = 24;
+
+  oneof o {
+    string oneof_str_field = 31;
+    int32 oneof_int32_field = 32;
+    TopLevelEnum oneof_enum_field = 33;
+    TopLevelMessage oneof_message_field = 34;
+  }
+
+}

Protos/pluginlib_descriptor_test2.proto

@@ -0,0 +1,55 @@
+// Protos/pluginlib_descriptor_test.proto - test proto
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2014 - 2020 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See http://swift.org/LICENSE.txt for license information
+// See http://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+// -----------------------------------------------------------------------------
+///
+/// Test proto for Tests/SwiftProtobufPluginLibraryTests/Test_Descriptor.swift
+///
+// -----------------------------------------------------------------------------
+
+syntax = "proto3";
+
+package swift_descriptor_test;
+
+message Proto3MessageForPresence {
+
+  enum SubEnum {
+    SUB_VALUE_0 = 0;
+    SUB_VALUE_1 = 1;
+    SUB_VALUE_2 = 2;
+  }
+
+  string str_field = 1;
+  int32 int32_field = 2;
+  SubEnum enum_field = 3;
+  OtherMessage message_field = 4;
+
+  optional string opt_str_field = 11;
+  optional int32 opt_int32_field = 12;
+  optional SubEnum opt_enum_field = 13;
+  optional OtherMessage opt_message_field = 14;
+
+  repeated string repeat_str_field = 21;
+  repeated int32 repeat_int32_field = 22;
+  repeated SubEnum repeat_enum_field = 23;
+  repeated OtherMessage repeat_message_field = 24;
+
+  oneof o {
+    string oneof_str_field = 31;
+    int32 oneof_int32_field = 32;
+    SubEnum oneof_enum_field = 33;
+    OtherMessage oneof_message_field = 34;
+  }
+
+}
+
+message OtherMessage {
+  string field = 1;
+}