Add constexpr to <iterator> and <array> for C++17
* include/bits/c++config (_GLIBCXX17_CONSTEXPR): Define.
* include/bits/range_access.h (begin, end, rbegin, rend, crbegin)
(crend): Add _GLIBCXX17_CONSTEXPR as per P0031R0.
* include/bits/stl_iterator.h (reverse_iterator, move_iterator)
(__make_reverse_iterator, make_reverse_iterator, make_move_iterator):
Likewise.
* include/bits/stl_iterator_base_funcs.h (__distance, __advance): Add
_GLIBCXX14_CONSTEXPR.
(distance, advance, next, prev): Add _GLIBCXX17_CONSTEXPR.
* include/std/array (array::begin, array::end, array::rbegin)
(array::rend, array::cbegin, array:cend, array::crbegin)
(array::crend, array::operator[], array::at, array::front)
(array::back, array::data): Likewise.
* testsuite/24_iterators/headers/iterator/range_access.cc: Replace
with separate tests for C++11, C++14, and C++17.
* testsuite/24_iterators/headers/iterator/range_access_c++11.cc: New.
* testsuite/24_iterators/headers/iterator/range_access_c++14.cc: New.
* testsuite/24_iterators/headers/iterator/range_access_c++17.cc: New.
From-SVN: r239690
06db992005