libstdc++: Skip redundant assertions in std::span construction [PR117966]

libstdc++: Skip redundant assertions in std::span construction [PR117966]

As PR c++/117966 shows, the Debug Mode checks cause a compilation error
for a global constexpr std::span. Those debug checks are redundant when
constructing from an array or a range, because we already know we have a
valid range and we know its size. Instead of delegating to the
std::span(contiguous_iterator, contiguous_iterator) constructor, just
initialize the data members directly.

libstdc++-v3/ChangeLog:

	PR libstdc++/117966
	* include/std/span (span(T (&)[N])): Do not delegate to
	constructor that performs redundant checks.
	(span(array<T, N>&), span(const array<T, N>&)): Likewise.
	(span(Range&&), span(const span<T, N>&)): Likewise.
	* testsuite/23_containers/span/117966.cc: New test.

(cherry picked from commit e95bda027e0b81922c1bf44770674190bdf787e8)