libstdc++: Fix _ForwardIteratorConcept for __gnu_debug::vector<bool>

libstdc++: Fix _ForwardIteratorConcept for __gnu_debug::vector<bool>

The recent changes to the _GLIBCXX_CONCEPT_CHECKS checks for forward
iterators don't work for vector<bool> iterators in debug mode, because
the _Safe_iterator specializations don't match the special cases I added
for _Bit_iterator and _Bit_const_iterator.

This refactors the _ForwardIteratorReferenceConcept class template to
identify vector<bool> iterators using a new trait, which also works for
debug iterators.

libstdc++-v3/ChangeLog:

	* include/bits/boost_concept_check.h (_Is_vector_bool_iterator):
	New trait to identify vector<bool> iterators, including debug
	ones.
	(_ForwardIteratorReferenceConcept): Add default template
	argument using _Is_vector_bool_iterator and use it in partial
	specialization for the vector<bool> cases.
	(_Mutable_ForwardIteratorReferenceConcept): Likewise.
	* testsuite/24_iterators/operations/prev_neg.cc: Adjust dg-error
	line number.