libstdc++: Implement string_view range constructor for C++20

libstdc++: Implement string_view range constructor for C++20

This implements the new string_view constructor proposed by P1989R2.
This hasn't been voted into the C++23 draft yet, but it's been reviewed
by LWG and is expected to be approved at the next WG21 meeting.

libstdc++-v3/ChangeLog:

	* include/std/string_view (basic_string_view(Range&&)): Define new
	constructor and deduction guide.
	* testsuite/21_strings/basic_string_view/cons/char/range_c++20.cc: New test.
	* testsuite/21_strings/basic_string_view/cons/wchar_t/range_c++20.cc: New test.