C front-end: report struct/union redefinition as an error

We previously left this to a failing invariant in namespacet::follow_tag
to be reported. Instead, provide a meaningful error message.

Test generated using C-Reduce starting from an SV-COMP task.

Author: tautschnig

regression/ansi-c/struct8/main.c

@@ -0,0 +1,2 @@
+struct a;
+union a b;

regression/ansi-c/struct8/test.desc

@@ -0,0 +1,9 @@
+CORE test-c++-front-end
+main.c
+
+^EXIT=(64|1)$
+^SIGNAL=0$
+redefinition of '(struct )?a' as different kind of tag
+^CONVERSION ERROR$
+--
+Invariant check failed

src/ansi-c/c_typecheck_type.cpp

@@ -847,6 +847,13 @@ void c_typecheck_baset::typecheck_compound_type(struct_union_typet &type)
           symbol_table.get_writeable_ref(s_it->first).type.swap(type);
         }
       }
+      else if(s_it->second.type.id() != type.id())
+      {
+        error().source_location = type.source_location();
+        error() << "redefinition of '" << s_it->second.pretty_name << "'"
+                << " as different kind of tag" << eom;
+        throw 0;
+      }
       else if(have_body)
       {
         error().source_location=type.source_location();

src/cpp/cpp_typecheck_compound_type.cpp

@@ -212,6 +212,13 @@ void cpp_typecheckt::typecheck_compound_type(
         throw 0;
       }
     }
+    else if(symbol.type.id() != type.id())
+    {
+      error().source_location = type.source_location();
+      error() << "redefinition of '" << symbol.pretty_name << "'"
+              << " as different kind of tag" << eom;
+      throw 0;
+    }
   }
   else
   {