Check precondition for std::ssize(const Container&)

Check precondition for std::ssize(const Container&)

This precondition is the subject of a national body comment on the C++20
CD. This just adds a test to ensure we enforce the precondition.

Also move existing 24_iterator/range_access*.cc tests to a dedicated
directory for the [iterator.range] subclause.

	* testsuite/24_iterators/range_access*.cc: Move to ...
	* testsuite/24_iterators/range_access/range_access*.cc: ... here.
	* testsuite/24_iterators/range_access/range_access_cpp20_neg.cc: New
	test.

From-SVN: r277658