C front-end: support alias attributes on variables

GCC only documents those for functions, but the Linux kernel code uses aliases
when building kernel modules. These result from the expansion of the following
macro defined in mdoule.h:

 #define MODULE_DEVICE_TABLE(type, name)         \
 extern typeof(name) __mod_##type##__##name##_device_table   \
   __attribute__ ((unused, alias(__stringify(name))))

Author: tautschnig

regression/ansi-c/gcc_attributes12/main.c

@@ -0,0 +1,9 @@
+static const int my_ids[] = { 1, 2, 3, 4 };
+
+#ifdef __GNUC__
+extern typeof(my_ids) my_ids_table __attribute__((alias("my_ids")));
+#endif
+
+int main()
+{
+}

regression/ansi-c/gcc_attributes12/test.desc

@@ -0,0 +1,8 @@
+CORE
+main.c
+
+^EXIT=0$
+^SIGNAL=0$
+--
+^warning: ignoring
+^CONVERSION ERROR$

src/ansi-c/c_typecheck_base.cpp

@@ -152,7 +152,7 @@ void c_typecheck_baset::typecheck_new_symbol(symbolt &symbol)
         it->set_identifier(irep_idt());
     }
   }
-  else
+  else if(!symbol.is_macro)
   {
     // check the initializer
     do_initializer(symbol);