Keep track of types for structs and unions

When dealing withs structs (and unions) we replace the type with a
symbol for that type (that may or may not be new). However, if a struct
is declared, we still need to track any qualifiers (e.g. a variable that
is introduced as a const struct).

Author: thk123

src/ansi-c/c_typecheck_type.cpp

@@ -820,7 +820,9 @@ void c_typecheck_baset::typecheck_compound_type(struct_union_typet &type)
   symbol_type.add_source_location()=type.source_location();
   symbol_type.set_identifier(identifier);
 
+  c_qualifierst original_qualifiers(type);
   type.swap(symbol_type);
+  original_qualifiers.write(type);
 }
 
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