libstdc++: Fix function that can't be constexpr in C++11 (PR 95289)

libstdc++: Fix function that can't be constexpr in C++11 (PR 95289)

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.