Move namespace ranges inside the feature test macro guard, because
'ranges' is not a reserved name before C++20.
libstdc++-v3/ChangeLog:
* include/std/numeric (ranges): Only declare namespace for C++23
and later.
(ranges::iota_result): Fix indentation.
* testsuite/17_intro/names.cc: Check ranges is not used as an
identifier before C++20.