libstdc++: Implement P1872R0 and P1394R0 for std::span

libstdc++: Implement P1872R0 and P1394R0 for std::span

This also fixes a bug in the implementation of LWG 3255, which causes:
FAIL: 23_containers/span/lwg3255.cc (test for excess errors)
That's because the test was wrong and verified the buggy behaviour. That
will be fixed in the following commit.

2019-12-05  JeanHeyd "ThePhD" Meneide  <phdofthehouse@gmail.com>

	Implement P1872R0 and P1394R0 for std::span
	* include/bits/range_access.h (__adl_begin, __adl_end): Remove.
	(sentinel_t, range_value_t, range_reference_t)
	(range_rvalue_reference_t, __forwarding_range, disable_sized_range)
	(output_range, input_range, forward_range, bidirectional_range)
	(random_access_range, contiguous_range, common_range): Move here from
	<ranges>, to make this the "ranges lite" internal header.
	* include/std/ranges: Move basic aliases and concepts to
	<bits/range_access.h>.
	* include/std/span: Use concepts and ranges:: calls instead of
	enable_if and friends.
	* include/std/type_traits: Add __is_array_convertible trait.

From-SVN: r279000