Disallow empty structs

test/libsolidity/SolidityNameAndTypeResolution.cpp

@@ -7279,7 +7279,7 @@ BOOST_AUTO_TEST_CASE(modifiers_access_storage_pointer)
 {
 	char const* text = R"(
 		contract C {
-			struct S { }
+			struct S { uint a; }
 			modifier m(S storage x) {
 				x;
 				_;

test/libsolidity/syntaxTests/empty_struct.sol

@@ -0,0 +1,5 @@
+contract test {
+	struct A {}
+}
+// ----
+// SyntaxError: Defining empty structs is disallowed.