Add namespacet::follow_struct_union_tag to simplify follow_tag uses

Replacing namespacet::follow by namespacet::follow_tag produced several
instances that require expanding composite data types, without a need to
distinguish structs vs unions.

Author: tautschnig

Diff for: src/util/namespace.cpp

@@ -92,6 +92,15 @@ namespace_baset::follow_tag(const c_enum_tag_typet &src) const
   return to_c_enum_type(symbol.type);
 }
 
+const struct_union_typet &follow_struct_union_tag(const tag_typet &src) const
+{
+  PRECONDITION(src.id() == ID_struct_tag || src.id() == ID_union_tag);
+  const symbolt &symbol = lookup(src.get_identifier());
+  CHECK_RETURN(symbol.is_type);
+  CHECK_RETURN(symbol.type.id() == ID_struct || symbol.type.id() == ID_union);
+  return to_struct_union_type(symbol.type);
+}
+
 /// Follow macros to their values in a given expression.
 /// \param expr: The expression to follow macros in.
 void namespace_baset::follow_macros(exprt &expr) const

Diff for: src/util/namespace.h

@@ -68,6 +68,9 @@ class namespace_baset
   const struct_typet &follow_tag(const struct_tag_typet &) const;
   const c_enum_typet &follow_tag(const c_enum_tag_typet &) const;
 
+  /// Resolve a `struct_tag_typet` or `union_tag_typet` to the complete version.
+  const struct_union_typet &follow_struct_union_tag(const tag_typet &) const;
+
   /// Returns the minimal integer n such that there is no symbol (in any of the
   /// symbol tables) whose name is of the form "An" where A is \p prefix.
   /// The intended use case is finding the next available symbol name for a