libstdc++: Implement N3644 on _Safe_iterator<> [PR114316]

Consider range of value-initialized iterators as valid and empty.

libstdc++-v3/ChangeLog:

	PR libstdc++/114316
	* include/debug/safe_iterator.tcc (_Safe_iterator<>::_M_valid_range):
	First check if both iterators are value-initialized before checking if
	singular.
	* testsuite/23_containers/set/debug/114316.cc: New test case.
	* testsuite/23_containers/vector/debug/114316.cc: New test case.