PR libstdc++/28080 (partial)
* include/std/bitset: Do not include <istream> and <ostream>,
<ios> is enough.
* include/std/iomanip: Do not include the whole <istream>,
<ios> is enough; do not include <functional>; reformat.
* include/std/functional: Do not include <cstddef>.
* include/std/list: Do not include <bits/stl_uninitialized.h>.
* include/std/numeric: Do not include the whole <iterator> and
<bits/stl_function.h>.
* include/std/valarray: Do not include <numeric>.
* include/bits/stl_numeric.h: Include <bits/concept_check.h>.
* src/iostream-inst.cc: Include <istream>.
* include/ext/functional: Include <cstddef>.
* testsuite/util/testsuite_hooks.h: Do not include <locale>,
<ctime> is enough; qualify tm.
* testsuite/util/testsuite_hooks.cc: Qualify tm.