(Regression, caused by a patch for the bug 22646).
Problem: when result type of date_format() was changed from
binary string to character string, mixing date_format()
with a ascii column in CONCAT() stopped to work.
Fix:
- adding "repertoire" flag into DTCollation class,
to mark items which can return only pure ASCII strings.
- allow character set conversion from pure ASCII to other character sets.


include/m_ctype.h:
  Defining new flags.
  Adding new function prototypes.
mysql-test/r/ctype_ucs.result:
  Adding tests.
mysql-test/r/ctype_utf8.result:
  Adding tests.
mysql-test/r/func_time.result:
  Adding tests.
mysql-test/t/ctype_ucs.test:
  Adding tests.
mysql-test/t/ctype_utf8.test:
  Adding tests.
mysql-test/t/func_time.test:
  Adding test.
mysys/charset.c:
  Adding pure ASCII detection when loading a dynamic character set.
sql/item.cc:
  - Moving detection of a Unicode superset into function.
  - Adding detection of a ASCII subset.
  - Adding creation of to-ASCII character set convertor when
    safe_charset_converter() failed and when the argument.
    repertoire is know to be pure ASCII.
sql/item.h:
  - Adding "repertoire" member into DTCollation class.
  - Adding "repertoire" argument to constructors.
  - Adding new methods:
    set_repertoire_from_charset()
    set_repertoire_from_value()
sql/item_func.cc:
  Adding "repertoire" argument.
sql/item_strfunc.cc:
  Adding "repertoire" argument.
sql/item_timefunc.cc:
  Initializing the result repertoire taking into account the "is_ascii"
  flag of the current locale.
sql/sql_lex.cc:
  Detect 7bit strings, return in Lex->text_string_is_7bit.
sql/sql_lex.h:
  Adding new member into LEX structure.
  Adding new member into Lex_input_stream
sql/sql_string.cc:
  Allow simple copy from pure ASCII to a ASCII-based character set.
sql/sql_yacc.yy:
  Depening on Lex->text_string_is_7bit and character set features,
  create Item_string with MY_REPERTOIRE_ASCII when it is possible.
strings/conf_to_src.c:
  - Adding printing of the "MY_CS_PUREASCII" flag
  - Adding printing of copyright
strings/ctype-extra.c:
  Recreating ctype-extra.c: ascii_general_ci and ascii_bin
  are now marked with MY_CS_PUREASCII flag.
strings/ctype.c:
  Adding new functions.