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.

This library issue hasn't been resolved yet, but the proposed
resolution looks right and so I'm implementing it now.

Tested powerpc64le-linux, committed to trunk.