Mark further field sensitivity methods "nodiscard"

All methods are about their return values, not their side effects. This
is to make sure we catch potential usage errors at compile time.

Author: tautschnig

src/goto-symex/field_sensitivity.h

@@ -128,6 +128,7 @@ class field_sensitivityt
   apply(const namespacet &ns, goto_symex_statet &state, exprt expr, bool write)
     const;
   /// \copydoc apply(const namespacet&,goto_symex_statet&,exprt,bool) const
+  NODISCARD
   exprt apply(
     const namespacet &ns,
     goto_symex_statet &state,
@@ -142,6 +143,7 @@ class field_sensitivityt
   /// \return Expanded expression; for example, for a \p ssa_expr of some struct
   ///   type, a `struct_exprt` with each component now being an SSA expression
   ///   is built.
+  NODISCARD
   exprt get_fields(
     const namespacet &ns,
     goto_symex_statet &state,
@@ -172,6 +174,7 @@ class field_sensitivityt
     symex_targett &target,
     bool allow_pointer_unsoundness);
 
+  NODISCARD
   exprt simplify_opt(exprt e, const namespacet &ns) const;
 };