libstdc++: Pretty printers for _Bit_reference and _Bit_iterator

libstdc++: Pretty printers for _Bit_reference and _Bit_iterator

'std::_Bit_iterator' and 'std::_Bit_const_iterator' are the iterators
used by 'std::vector<bool>'.
'std::_Bit_reference' is e.g. used in range-based for loops over
'std::vector<bool>'  like

    std::vector<bool> vb {true, false, false};
    for (auto b : vb) {
        // b is of type std::_Bit_reference here
        // ...
    }

Like iterators of vectors for other types, the actual value is printed.

libstdc++-v3/ChangeLog:

	* python/libstdcxx/v6/printers.py (StdBitIteratorPrinter)
	(StdBitReferencePrinter): Add pretty-printers for
	_Bit_reference, _Bit_iterator and _Bit_const_iterator.
	* testsuite/libstdc++-prettyprinters/simple.cc: Test
	std::_Bit_reference, std::_Bit_iterator and
	std::_Bit_const_iterator.
	* testsuite/libstdc++-prettyprinters/simple11.cc: Likewise.