Visible function type RTTI for Clang -fsanitize=function

--- boost/function/function_base.hpp
+++ boost/function/function_base.hpp
@@ -159,7 +159,7 @@
       };
 
       // The operation type to perform on the given functor/function pointer
-      enum functor_manager_operation_type {
+      enum BOOST_SYMBOL_VISIBLE functor_manager_operation_type {
         clone_functor_tag,
         move_functor_tag,
         destroy_functor_tag,