libstdc++: Do not expose set_brackets/set_separator for formatter with format_kind...

libstdc++: Do not expose set_brackets/set_separator for formatter with format_kind other than sequence [PR119861]

The standard defines separate specializations of range-default-formatter, out
of which only one for range_format::sequence provide the set_brackets and
set_separator methods. We implemented it as one specialization and exposed
this method for range_format other than string or debug_string, i.e. when
range_formatter was used as underlying formatter.

	PR libstdc++/119861

libstdc++-v3/ChangeLog:

	* include/std/format (formatter<_Rg, _CharT>::set_separator)
	(formatter<_Rg, _CharT>::set_brackets): Constrain with
	(format_kind<_Rg> == range_format::sequence).
	* testsuite/std/format/ranges/pr119861_neg.cc: New test.