Switch Member to variant in anticipation of more kinds. (#656)

Note this overlaps a little with #644's Member changes, but not too badly.

executable_semantics/ast/declaration.cpp

@@ -13,7 +13,7 @@ void FunctionDeclaration::Print() const { definition.Print(); }
 void StructDeclaration::Print() const {
   std::cout << "struct " << *definition.name << " {" << std::endl;
   for (auto& member : *definition.members) {
-    PrintMember(member);
+    member->Print();
   }
   std::cout << "}" << std::endl;
 }

executable_semantics/ast/member.cpp

@@ -8,21 +8,24 @@
 
 namespace Carbon {
 
-auto MakeField(int line_num, std::string name, const Expression* type)
-    -> Member* {
+auto Member::MakeFieldMember(int line_num, std::string name,
+                             const Expression* type) -> Member* {
   auto m = new Member();
   m->line_num = line_num;
-  m->tag = MemberKind::FieldMember;
-  m->u.field.name = new std::string(std::move(name));
-  m->u.field.type = type;
+  m->value = FieldMember({.name = std::move(name), .type = type});
   return m;
 }
 
-void PrintMember(Member* m) {
-  switch (m->tag) {
+auto Member::GetFieldMember() const -> const FieldMember& {
+  return std::get<FieldMember>(value);
+}
+
+void Member::Print() {
+  switch (tag()) {
     case MemberKind::FieldMember:
-      std::cout << "var " << *m->u.field.name << " : ";
-      PrintExp(m->u.field.type);
+      const auto& field = GetFieldMember();
+      std::cout << "var " << field.name << " : ";
+      PrintExp(field.type);
       std::cout << ";" << std::endl;
       break;
   }

executable_semantics/ast/member.h

@@ -13,21 +13,29 @@ namespace Carbon {
 
 enum class MemberKind { FieldMember };
 
-struct Member {
-  int line_num;
-  MemberKind tag;
-  union {
-    struct {
-      std::string* name;
-      const Expression* type;
-    } field;
-  } u;
+struct FieldMember {
+  static constexpr MemberKind Kind = MemberKind::FieldMember;
+  std::string name;
+  const Expression* type;
 };
 
-auto MakeField(int line_num, std::string name, const Expression* type)
-    -> Member*;
+struct Member {
+  static auto MakeFieldMember(int line_num, std::string name,
+                              const Expression* type) -> Member*;
 
-void PrintMember(Member* m);
+  auto GetFieldMember() const -> const FieldMember&;
+
+  void Print();
+
+  inline auto tag() const -> MemberKind {
+    return std::visit([](const auto& t) { return t.Kind; }, value);
+  }
+
+  int line_num;
+
+ private:
+  std::variant<FieldMember> value;
+};
 
 }  // namespace Carbon