MDEV-33772 Bad SEPARATOR value in GROUP_CONCAT on character set conversion

Item_func_group_concat::print() did not take into account
that Item_func_group_concat::separator can be of a different character set
than the "String *str" (when the printing is being done to).
Therefore, printing did not work correctly for:
- non-ASCII separators when GROUP_CONCAT is done on 8bit data
  or multi-byte data with mbminlen==1.
- all separators (even including simple ones like comma)
  when GROUP_CONCAT is done on ucs2/utf16/utf32 data (mbminlen>1).

Because of this problem, VIEW definitions did not print correctly to
their FRM files. This later led to a wrong SELECT and SHOW CREATE output.

Fix:

- Adding new String methods:

  bool append_for_single_quote_using_mb_wc(const char *str, size_t length,
                                           CHARSET_INFO *cs);

  bool append_for_single_quote_opt_convert(const char *str,
                                           size_t length,
                                           CHARSET_INFO *cs)

  which perform both escaping and character set conversion at the same time.

- Adding a new String method escaped_wc_for_single_quote(),
  to reuse the code between the old and the new methods.

- Fixing Item_func_group_concat::print() to use the new
  method append_for_single_quote_opt_convert().

mysql-test/main/func_gconcat.test

@@ -1105,4 +1105,20 @@ set group_concat_max_len=default;
 
 drop table t1,t2;
 
+--echo #
+--echo # MDEV-33772 Bad SEPARATOR value in GROUP_CONCAT on character set conversion
+--echo #
+
+SET NAMES utf8, @@collation_connection=latin1_swedish_ci;
+CREATE TABLE t1 (c VARCHAR(10)) CHARACTER SET latin1;
+INSERT INTO t1 VALUES ('a'),('A');
+CREATE OR REPLACE VIEW v1 AS
+  SELECT GROUP_CONCAT(c SEPARATOR 'ß') AS c1 FROM t1 GROUP BY c;
+SELECT * FROM v1;
+SELECT HEX(c1) FROM v1;
+SHOW CREATE VIEW v1;
+DROP VIEW v1;
+DROP TABLE t1;
+SET NAMES latin1;
+
 --echo # End of 10.5 tests