C++ front-end: support attributes with tag-less structs

Seen in GCC's STL.

Author: tautschnig

regression/cpp/gcc_attributes2/main.cpp

@@ -1,6 +1,11 @@
 #ifdef __GNUC__
 typedef int my_int16_t __attribute__((__mode__(__HI__)));
 static_assert(sizeof(my_int16_t) == 2, "16 bit");
+
+template <std::size_t _Align = __alignof__(int)>
+struct __attribute__((__aligned__((_Align))))
+{
+} __align;
 #endif
 
 int main()

src/cpp/parse.cpp

@@ -4486,6 +4486,12 @@ bool Parser::rClassSpec(typet &spec)
   std::cout << std::string(__indent, ' ') << "Parser::rClassSpec 3\n";
 #endif
 
+  if(!optAlignas(spec))
+    return false;
+
+  if(!optAttribute(spec))
+    return false;
+
   if(lex.LookAhead(0)=='{')
   {
     // no tag
@@ -4495,12 +4501,6 @@ bool Parser::rClassSpec(typet &spec)
   }
   else
   {
-    if(!optAlignas(spec))
-      return false;
-
-    if(!optAttribute(spec))
-      return false;
-
     irept name;
 
     if(!rName(name))