C++ type conversion: gcc_attribute_mode must not be re-converted

C typecheck_type performs its own conversion of raw type structures, and
can't deal with those already done by the C++ front-end. Therefore,
restored the "merged_type" before handing off to the C front-end.

Also clean up unnecessary enum conversion steps, which later type
checking stages actually take care of.

Author: tautschnig

regression/ansi-c/gcc_attributes6/test.desc

@@ -1,4 +1,4 @@
-CORE gcc-only
+CORE gcc-only test-c++-front-end
 main.c
 
 ^EXIT=0$

src/cpp/cpp_convert_type.cpp

@@ -330,15 +330,10 @@ void cpp_convert_plain_type(typet &type, message_handlert &message_handler)
     type.id() == ID_union || type.id() == ID_array || type.id() == ID_code ||
     type.id() == ID_unsignedbv || type.id() == ID_signedbv ||
     type.id() == ID_bool || type.id() == ID_floatbv || type.id() == ID_empty ||
-    type.id() == ID_constructor || type.id() == ID_destructor)
+    type.id() == ID_constructor || type.id() == ID_destructor ||
+    type.id() == ID_c_enum)
   {
   }
-  else if(type.id()==ID_c_enum)
-  {
-    // add width -- we use int, but the standard
-    // doesn't guarantee that
-    type.set(ID_width, config.ansi_c.int_width);
-  }
   else if(type.id() == ID_c_bool)
   {
     type.set(ID_width, config.ansi_c.bool_width);

src/cpp/cpp_typecheck_type.cpp

@@ -9,15 +9,15 @@ Author: Daniel Kroening, kroening@cs.cmu.edu
 /// \file
 /// C++ Language Type Checking
 
-#include "cpp_typecheck.h"
-
-#include <util/source_location.h>
-#include <util/simplify_expr.h>
 #include <util/c_types.h>
+#include <util/simplify_expr.h>
+#include <util/source_location.h>
 
 #include <ansi-c/c_qualifiers.h>
+#include <ansi-c/merged_type.h>
 
 #include "cpp_convert_type.h"
+#include "cpp_typecheck.h"
 #include "cpp_typecheck_fargs.h"
 
 void cpp_typecheckt::typecheck_type(typet &type)
@@ -274,6 +274,13 @@ void cpp_typecheckt::typecheck_type(typet &type)
   }
   else if(type.id() == ID_gcc_attribute_mode)
   {
+    PRECONDITION(type.has_subtype());
+    merged_typet as_parsed;
+    as_parsed.move_to_subtypes(type.subtype());
+    type.get_sub().clear();
+    as_parsed.move_to_subtypes(type);
+    type.swap(as_parsed);
+
     c_typecheck_baset::typecheck_type(type);
   }
   else if(type.id() == ID_complex)