Fix -Wunused warnings in libstdc++ headers

Fix -Wunused warnings in libstdc++ headers

	* config/io/basic_file_stdio.h (__basic_file): Remove name of unused
	parameter.
	* include/bits/boost_concept_check.h: Add pragmas to disable
	-Wunused-local-typedef warnings.
	* include/bits/codecvt.h (codecvt_byname<char16_t, char, mbstate_t>)
	(codecvt_byname<char32_t, char, mbstate_t>): Remove name of unused
	parameter.
	* include/bits/locale_facets_nonio.tcc (time_get::do_get_weekday)
	(time_get::do_get_monthname, time_get::do_get_year): Remove unused
	variables.
	* include/std/bitset (_Base_bitset<0>::_M_getword): Remove name of
	unused parameter.
	* include/std/streambuf (_IsUnused): Define.
	(basic_streambuf::imbue, basic_streambuf::pbackfail)
	(basic_streambuf::overflow): Add macro to unused parameters.
	* testsuite/24_iterators/operations/prev_neg.cc: Adjust dg-error.

From-SVN: r255391