Implement std::span for C++20

Implement std::span for C++20

2019-09-05  JeanHeyd Meneide  <phdofthehouse@gmail.com>

	* include/Makefile.am: Add <span> header.
	* include/Makefile.in: Regenerate.
	* include/bits/range_access.h (__adl_begin, __adl_end, __adl_cbegin)
	(__adl_cend, __adl_rbegin, __adl_rend, __adl_crbegin, __adl_crend)
	(__adl_data, __adl_cdata, __adl_size, __adl_empty, __adl_to_address):
	New functions for performing argument-dependent lookup of range
	customization points.
	* include/bits/stl_iterator.h (__normal_iterator): Add
	_GLIBCXX20_CONSTEXPR to all functions.
	* include/std/span: New header.
	* include/std/version (__cpp_lib_span): Define feature test macro.
	* testsuite/23_containers/span/contiguous_range_neg.cc: New test.
	* testsuite/23_containers/span/everything.cc: New test.
	* testsuite/23_containers/span/get_neg.cc: New test.
	* testsuite/23_containers/span/last_neg.cc: New test.
	* testsuite/23_containers/span/subspan_neg.cc: New test.
	* testsuite/23_containers/span/tuple_element_dynamic_neg.cc: New test.
	* testsuite/23_containers/span/tuple_element_oob_neg.cc: New test.
	* testsuite/23_containers/span/tuple_size_neg.cc: New test.

From-SVN: r275403