This C++17 header is supported in C++14 as a GNU extension, but stopped
working last year because I made it depend on an internal helper which
is only defined for C++17 and up.
PR libstdc++/94520
* include/std/charconv (__integer_to_chars_result_type)
(__integer_from_chars_result_type): Use __or_ instead of __or_v_ to
allow use in C++14.
* testsuite/20_util/from_chars/1.cc: Run test as C++14 and replace
use of std::string_view with std::string.
* testsuite/20_util/from_chars/2.cc: Likewise.
* testsuite/20_util/to_chars/1.cc: Likewise.
* testsuite/20_util/to_chars/2.cc: Likewise.
