Implement P0426R1 "Constexpr for std::char_traits" for C++17 (partial)
* include/bits/char_traits.h (__gnu_cxx::char_traits): Add
_GLIBCXX14_CONSTEXPR on assign, compare, find, and length.
(std::char_traits<char>, std::char_traits<wchar_t>): Add
_GLIBCXX17_CONSTEXPR on assign.
(std::char_traits<char16_t>, std::char_traits<char32_t>): Add
_GLIBCXX17_CONSTEXPR on assign, compare, find, and length.
* testsuite/21_strings/char_traits/requirements/
constexpr_functions_c++17.cc: New test.
From-SVN: r246655
8c3b5c719f