Also fix the constraints on span(Container&) and span(const Container&)
constructors so that they aren't used for const spans or const arrays.
* include/std/span (span(element_type(&)[N]))
(span(array<value_type, N>&), span(const array<value_type, N>&)):
Deduce array element type to allow safe const conversions (LWG 3255).
[!_GLIBCXX_P1394] (span(Container&), span(const Container&)): Use
remove_cv_t on arguments to __is_std_span and __is_std_array.
* testsuite/23_containers/span/lwg3255.cc: New test.
From-SVN: r276298
582c57a17e