C front-end: address-of-label is a compile-time constant

When using computed gotos, the addresses of labels are taken. These are
compile-time constants that are safe to use in initializers of objects
with static lifetime.

Author: tautschnig

regression/cbmc/Computed-Goto1/main.c

@@ -1,6 +1,6 @@
 int main()
 {
-  void *table[]={ &&l0, &&l1, &&l2 };
+  static void *table[] = {&&l0, &&l1, &&l2};
   int in, out;
 
   if(in>=0 && in<=2)

src/ansi-c/c_typecheck_expr.cpp

@@ -4520,6 +4520,8 @@ class is_compile_time_constantt : public is_constantt
     }
     else if(e.id() == ID_array && e.get_bool(ID_C_string_constant))
       return true;
+    else if(e.id() == ID_label)
+      return true;
     else
       return is_constantt::is_constant_address_of(e);
   }