Restore `isConstructor` in the legacy AST.

libsolidity/ast/ASTJsonConverter.cpp

@@ -336,6 +336,8 @@ bool ASTJsonConverter::visit(FunctionDefinition const& _node)
 		make_pair("implemented", _node.isImplemented()),
 		make_pair("scope", idOrNull(_node.scope()))
 	};
+	if (m_legacy)
+		attributes.emplace_back("isConstructor", _node.isConstructor());
 	setJsonNode(_node, "FunctionDefinition", std::move(attributes));
 	return false;
 }