The body of this function isn't just a return statement, so it can't be
constexpr until C++14.
Backport from mainline
2020-05-23 Jonathan Wakely <jwakely@redhat.com>
PR libstdc++/95289
* include/debug/helper_functions.h (__get_distance): Only declare
as a constexpr function for C++14 and up.
* testsuite/25_algorithms/copy/debug/95289.cc: New test.
