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().

sql/sql_string.h

@@ -1134,6 +1134,42 @@ public:
       print_with_conversion(to, cs);
   }
 
+  static my_wc_t escaped_wc_for_single_quote(my_wc_t ch)
+  {
+    switch (ch)
+    {
+    case '\\':   return '\\';
+    case '\0':   return '0';
+    case '\'':   return '\'';
+    case '\n':   return 'n';
+    case '\r':   return 'r';
+    case '\032': return 'Z';
+    }
+    return 0;
+  }
+
+  // Append for single quote using mb_wc/wc_mb Unicode conversion
+  bool append_for_single_quote_using_mb_wc(const char *str, size_t length,
+                                           CHARSET_INFO *cs);
+
+  // Append for single quote with optional mb_wc/wc_mb conversion
+  bool append_for_single_quote_opt_convert(const char *str,
+                                           size_t length,
+                                           CHARSET_INFO *cs)
+  {
+    return charset() == &my_charset_bin || cs == &my_charset_bin  ||
+           my_charset_same(charset(), cs) ?
+           append_for_single_quote(str, length) :
+           append_for_single_quote_using_mb_wc(str, length, cs);
+  }
+
+  bool append_for_single_quote_opt_convert(const String &str)
+  {
+    return append_for_single_quote_opt_convert(str.ptr(),
+                                               str.length(),
+                                               str.charset());
+  }
+
   bool append_for_single_quote(const char *st, size_t len);
   bool append_for_single_quote(const String *s)
   {