libstdc++: Define __cpp_lib_constexpr_string macro
As noted in
r11-1339-gb6ab9ecd550227684643b41e9e33a4d3466724d8 we define
a non-standard __cpp_lib_constexpr_char_traits feature test macro to
indicate support for P0426R1 and P1032R1. At some point last year the
__cpp_lib_constexpr_string macro was retconned to indicate support for
those papers. This adds the new macro (which we didn't previously
define, because it referred to P0980R1 "Making std::string constexpr"
which we don't support).
libstdc++-v3/ChangeLog:
* include/bits/basic_string.h (__cpp_lib_constexpr_string): Define.
* include/std/version (__cpp_lib_constexpr_string): Define.
* testsuite/21_strings/char_traits/requirements/constexpr_functions_c++17.cc:
Check for __cpp_lib_constexpr_string.
* testsuite/21_strings/char_traits/requirements/version.cc: New test.
(cherry picked from commit
3da80ed7efd582575e7850a403ce693ec882d087)