libstdc++: Fix memchr path in std::ranges::find for non-common range [PR115799]

The memchr optimization introduced in r15-1857 needs to advance the
start iterator instead of returning the sentinel.

libstdc++-v3/ChangeLog:

	PR libstdc++/115799
	* include/bits/ranges_util.h (__find_fn): Return iterator
	instead of sentinel.
	* testsuite/25_algorithms/find/constrained.cc: Check non-common
	contiguous sized range of char.