Update feature test macro, add deleted operators, update u8path

This patch increments the __cpp_lib_char8_t feature test macro, adds deleted
operator<< overloads for basic_ostream, and modifies u8path to accept
sequences of char8_t for both the C++17 implementation of std::filesystem, and
the filesystem TS implementation.

The implementation mechanism used for u8path differs between the C++17 and
filesystem TS implementations.  The changes to the former take advantage of
C++17 'if constexpr'.  The changes to the latter retain C++11 compatibility
and rely on tag dispatching.

2019-11-29  Tom Honermann  <tom@honermann.net>

	Update feature test macro, add deleted operators, update u8path
	* include/bits/c++config: Bumped the value of the __cpp_lib_char8_t
	feature test macro.
	* include/bits/fs_path.h (u8path): Modified u8path to accept sequences
	of char8_t.
	* include/experimental/bits/fs_path.h (u8path): Modified u8path to
	accept sequences of char8_t.
	* include/std/ostream: Added deleted overloads of wchar_t, char8_t,
	char16_t, and char32_t for ordinary and wide formatted character and
	string inserters.

From-SVN: r278856