libstdc++: Prevent -Wstringop-overread warning in std::deque [PR100516]

libstdc++: Prevent -Wstringop-overread warning in std::deque [PR100516]

The compiler warns about the loop in deque::_M_range_initialize because
it doesn't know that the number of nodes has already been correctly
sized to match the size of the input. Use __builtin_unreachable to tell
it that the loop will never be entered if the number of elements is
smaller than a single node.

libstdc++-v3/ChangeLog:

	PR libstdc++/100516
	* include/bits/deque.tcc (_M_range_initialize<ForwardIterator>):
	Add __builtin_unreachable to loop.
	* testsuite/23_containers/deque/100516.cc: New test.