libstdc++: Ensure std::span and std::string_view are trivially copyable (P2251R1)

libstdc++: Ensure std::span and std::string_view are trivially copyable (P2251R1)

The recently approved P2251R1 paper requires these types to be trivially
copyable. They always have been in libstdc++, but add tests to check it.

libstdc++-v3/ChangeLog:

	* testsuite/21_strings/basic_string_view/requirements/trivially_copyable.cc:
	New test.
	* testsuite/23_containers/span/trivially_copyable.cc: New test.