libstdc++: Do not use list-initialization in std::span members [PR120997]

libstdc++: Do not use list-initialization in std::span members [PR120997]

As the bug report shows, for span<const bool> the return statements of
the form `return {data(), count};` will use the new C++26 constructor,
span(initializer_list<element_type>).

Although the conversions from data() to bool and count to bool are
narrowing and should be ill-formed, in system headers the narrowing
diagnostics are suppressed. In any case, even if the compiler diagnosed
them as ill-formed, we still don't want the initializer_list constructor
to be used. We want to use the span(element_type*, size_t) constructor
instead.

Replace the braced-init-list uses with S(data(), count) where S is the
correct return type. We need to make similar changes in the C++26
working draft, which will be taken care of via an LWG issue.

libstdc++-v3/ChangeLog:

	PR libstdc++/120997
	* include/std/span (span::first, span::last, span::subspan): Do
	not use braced-init-list for return statements.
	* testsuite/23_containers/span/120997.cc: New test.