Add function is_native_function_with_warn()

There is a huge piece of code with huge comments in sql_yacc.yy:

if (is_native_function(thd, & sp->m_name))

{

  /*

   Huge comment

  */

  ...

}

responsible to display a warning when a user shadows a built-in function with a stored function.

The same warning will be needed for package routines (MDEV-10591).

We'll move this piece of code to sql_lex.cc into a new function is_native_function_with_warn().

This change was originally a part of MDEV-11952, but it was decided to commit it as a standalone change.