libstdc++: Fix and simplify constraints on std::span constructors

libstdc++: Fix and simplify constraints on std::span constructors

This makes the constraints consistent with the pre-Prague working paper.

	* include/std/span (span::__is_compatible_array): Simplify alias
	template by using requires-clause.
	(span::__is_compatible_ref): New alias template for constraining
	constructors.
	(span::__is_compatible_iterator, span::__is_compatible_range): Remove.
	(span(It, size_type), span(It, End)): Use __is_compatible_ref.
	(span(T(&)[N], span(array<T, N>&), span(const array<T, N>&)): Remove
	redundant parentheses.
	(span(R&&)): Add missing constraints.