libstdc++: Make ranges::size and ranges::empty check for unbounded arrays

libstdc++: Make ranges::size and ranges::empty check for unbounded arrays

Passing IncompleteType(&)[] to ranges::begin produces an error outside
the immediate context, which is fine for ranges::begin, but it means
that we fail to enforce the SFINAE-able constraints for ranges::size and
ranges::size. They should not be callable for any array of unknown
bound, whether the type is complete or not. Because we don't enforce
that in their constraints, we get a hard error when they try to use
ranges::begin.

This simply adds explicit checks for arrays of unknown bound to the
constraints for ranges::size and ranges::empty. We only need to check it
for the __sentinel_size and __eq_iter_empty concepts, because those are
the ones that are relevant to arrays, and which try to use
ranges::begin.

libstdc++-v3/ChangeLog:

	* include/bits/ranges_base.h (ranges::size, ranges::empty): Add
	explicit check for unbounded arrays before using ranges::begin.
	* testsuite/std/ranges/access/empty.cc: Check handling of unbounded
	arrays.
	* testsuite/std/ranges/access/size.cc: Likewise.