- always remove FC's internal attribute everywhere before deciding whether
  a cast is needed.
- ACSL and C decisions to cast are similar
- only unroll type for checking equality. If a cast is needed, keep
  typedef (if any) as target

- The global variable is now a Frama-C built-in and is not monitored;
- The `varinfo` for the variable, retrieved in module `Prepare_ast` is
  now exposed;
- The variable is added to every generated code so that it can be used
  by a later analysis of E-ACSL.

Avoid warning about missing proto during typechecking.
Also changes a warning into an error, and improves the error message.

Basile Desloges authored 5 years ago and Julien Signoles committed 5 years ago

A `file` parameter is added before `line`, and the `char *` parameters
have been converted to `const char *`.