Add Field::val_lex_string_strmake()

As of 11.3.0, the server code uses two ways to put a Field::val_str() value to MEM_ROOT:

char *get_field(MEM_ROOT *mem, Field *field);

bool get_field(MEM_ROOT *mem, Field *field, class String *res);

In many cases the value is further needed as a LEX_CSTRING, but both versions of get_field() are inconvenient and inefficient to initialize a LEX_CSTRING.

The first version requires an strlen() call. This is an example from udf_init() in sql_udf.cc:

    LEX_CSTRING name;

    name.str=get_field(&mem, table->field[0]);

    name.length = (uint) safe_strlen(name.str);

    char *dl_name= get_field(&mem, table->field[2]);

    ...

    if (!name.str || !dl_name || check_valid_path(dl_name, strlen(dl_name)) ||

Notice safe_strlen() and strlen() calls.

The second version of get_field() requires a String buffer for every LEX_CSTRING. This is an example from plugin_load() in sql_plugin.cc:

    String str_name, str_dl;

    get_field(tmp_root, table->field[0], &str_name);

    get_field(tmp_root, table->field[1], &str_dl);

    LEX_CSTRING name= {str_name.ptr(), str_name.length()};

    LEX_CSTRING dl=   {str_dl.ptr(), str_dl.length()};

Notice two String buffers.

Let's add a native method in Field which will overcome both problems:

LEX_STRING Field::val_lex_string_strmake(MEM_ROOT *mem)

{

  StringBuffer<MAX_FIELD_WIDTH> str;

  val_str(&str);

  char *to= strmake_root(mem, str.ptr(), str.length());

  return to ? LEX_STRING{to, str.length()} : LEX_STRING{NULL, 0};

}

It won't need neither strlen() calls nor String buffers.

Also let's move this version of get_field():

bool get_field(MEM_ROOT *mem, Field *field, String *res)

as a static function to sql_help.cc, as it's only used in this file. This will help to avoid reincarnation of its calls around the code in the future and encorage the use of the new method Field::val_lex_string_strmake().