Migrate Value to std::variant (#635)

Also:
- Switch data members, and associated parameters and return types to be values/references rather than pointers, where applicable.
- Move Expression's TagVisitor to anonymous namespace, to avoid name collision.
- Switch to trailing return syntax (per style guide).

executable_semantics/ast/expression.cpp

@@ -8,6 +8,21 @@
 
 namespace Carbon {
 
+namespace {
+
+struct TagVisitor {
+  template <typename Alternative>
+  auto operator()(const Alternative&) -> ExpressionKind {
+    return Alternative::Kind;
+  }
+};
+
+}  // namespace
+
+auto Expression::tag() const -> ExpressionKind {
+  return std::visit(TagVisitor(), value);
+}
+
 auto Expression::GetIdentifierExpression() const
     -> const IdentifierExpression& {
   return std::get<IdentifierExpression>(value);

executable_semantics/ast/expression.h

@@ -135,7 +135,7 @@ struct TypeTypeLiteral {
 
 struct Expression {
   int line_num;
-  inline auto tag() const -> ExpressionKind;
+  auto tag() const -> ExpressionKind;
 
   static auto MakeIdentifierExpression(int line_num, std::string var)
       -> const Expression*;
@@ -187,19 +187,6 @@ struct Expression {
 
 void PrintExp(const Expression* exp);
 
-// Implementation details only beyond this point
-
-struct TagVisitor {
-  template <typename Alternative>
-  auto operator()(const Alternative&) -> ExpressionKind {
-    return Alternative::Kind;
-  }
-};
-
-auto Expression::tag() const -> ExpressionKind {
-  return std::visit(TagVisitor(), value);
-}
-
 }  // namespace Carbon
 
 #endif  // EXECUTABLE_SEMANTICS_AST_EXPRESSION_H_