libstdc++: Use char for _Utf8_view if char8_t isn't available [PR114519]

libstdc++: Use char for _Utf8_view if char8_t isn't available [PR114519]

Instead of just omitting the definition of __unicode::_Utf8_view when
char8_t is disabled, we can make it use char instead.

libstdc++-v3/ChangeLog:

	PR libstdc++/114519
	* include/bits/unicode.h (_Utf8_view) [!__cpp_char8_t]: Define
	using char instead of char8_t.
	* testsuite/ext/unicode/view.cc: Use u8""sv literals to create
	string views, instead of std::u8string_view.