* include/ext/vstring_util.h: Do not include the whole <locale>.
* include/ext/vstring.tcc (operator>>(basic_istream<>&,
__versa_string<>&, getline(basic_istream<>&, __versa_string<>&,
_CharT)): Tweak to refer to ios_base as a base of istream; do not
refer to non-standard types of istream.
* include/bits/istream.tcc (operator>>(basic_istream<>&, _CharT*),
ws(basic_istream<>&)): Do not refer to non-standard types of istream.
* include/std/bitset (operator>>(std::basic_istream<>&, bitset<>&)):
Avoid using basic_streambuf<>*.
* include/bits/istream.tcc (operator>>(basic_istream<>&,
basic_string<>&), getline(basic_istream<>&, basic_string<>&, _CharT)):
Move...
* include/bits/basic_string.tcc: ... here; tweak to refer to ios_base
as a base of istream; do not refer to non-standard types of istream.
* include/std/string: Tweak includes.
* include/ext/type_traits.h (__is_null_pointer): Add.
* include/ext/rc_string_base.h: Use it.
* include/ext/sso_string_base.h: Likewise.
* include/bits/basic_string.tcc (__is_null_pointer): Remove, use
the above.
* include/ext/vstring_util.h (__vstring_utility<>::_S_is_null_pointer):
Remove.