Deprecate namespacet::follow

Follows struct and union tags, but does not follow enum tags. This is
not obvious from its documentation, and led to surprising behaviour as
seen (and fixed) in #8203. Using
suitable variants of `follow_tag` is safer.

Author: tautschnig

src/util/namespace.h

@@ -10,6 +10,7 @@ Author: Daniel Kroening, kroening@kroening.com
 #ifndef CPROVER_UTIL_NAMESPACE_H
 #define CPROVER_UTIL_NAMESPACE_H
 
+#include "deprecate.h"
 #include "invariant.h"
 #include "irep.h"
 
@@ -58,6 +59,7 @@ class namespace_baset
   virtual ~namespace_baset();
 
   void follow_macros(exprt &) const;
+  DEPRECATED(SINCE(2024, 2, 19, "use follow_tag(...) instead"))
   const typet &follow(const typet &) const;
 
   // These produce union_typet, struct_typet, c_enum_typet or