libstdc++: Allow explicit conversion of string views with different traits

This was changed by LWG 3857.

libstdc++-v3/ChangeLog:

	* include/std/string_view (basic_string_view(R&&)): Remove
	constraint that traits_type must be the same, as per LWG 3857.
	* testsuite/21_strings/basic_string_view/cons/char/range_c++20.cc:
	Explicit conversion between different specializations should be
	allowed.
	* testsuite/21_strings/basic_string_view/cons/wchar_t/range_c++20.cc:
	Likewise.