libstdc++ std::fill overload for std::vector<bool>::iterator

libstdc++ std::fill overload for std::vector<bool>::iterator

  Extend the overload so that it is used even when _GLIBCXX_DEBUG mode
is activated.

	* include/bits/stl_algobase.h (struct _Bit_iterator): New declaration.
	(std::__fill_a1(_Bit_iterator, _Bit_iterator, const bool&)): Likewise.
	* include/bits/stl_bvector.h (__fill_bvector): Move outside
	_GLIBCXX_STD_C namespace.
	(fill(_Bit_iterator, _Bit_iterator, const bool&)): Likewise and rename
	into...
	(__fill_a1): ...this.
	* testsuite/25_algorithms/fill/bvector/1.cc: New.