libstdc++: Implement reverse iteration for _Utf_view

libstdc++: Implement reverse iteration for _Utf_view

This implements the missing functions in _Utf_iterator to support
reverse iteration. All existing tests pass when the view is reversed, so
that the same code units are seen when iterating forwards or backwards.

libstdc++-v3/ChangeLog:

	* include/bits/unicode.h (_Utf_iterator::operator--): Reorder
	conditions and update position after reading a code unit.
	(_Utf_iterator::_M_read_reverse): Define.
	(_Utf_iterator::_M_read_utf8): Return extracted code point.
	(_Utf_iterator::_M_read_reverse_utf8): Define.
	(_Utf_iterator::_M_read_reverse_utf16): Define.
	(_Utf_iterator::_M_read_reverse_utf32): Define.
	* testsuite/ext/unicode/view.cc: Add checks for reversed views
	and reverse iteration.

Reviewed-by: Tomasz Kamiński <tkaminsk@redhat.com>