span: (almost) match std::span's constructor behavior

c++20's draft of std::span no longer includes move constructors.

span: add lifetimebound attribute

See http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p0936r0.pdf for
reference.

This helps to guard against dangling references caused by construction from
temporaries such as:

`Span<const int> sp(std::vector<int>{1,2,3});`

Add lifetimebound to attributes for general-purpose usage

Co-authored-by: practicalswift <practicalswift@users.noreply.github.com>

This is a backport of core#19387

With clang and gcc:
ninja all check-all