libstdc++: Fix experimental::filesystem::u8path(const Source&) for Windows

libstdc++: Fix experimental::filesystem::u8path(const Source&) for Windows

This function failed to compile when called with a std::string.

Also, constructing a path with a char8_t string did not correctly treat
the string as already UTF-8 encoded.

	* include/bits/fs_path.h (u8path(InputIterator, InputIterator))
	(u8path(const Source&)) [_GLIBCXX_FILESYSTEM_IS_WINDOWS]: Simplify
	conditions.
	* include/experimental/bits/fs_path.h [_GLIBCXX_FILESYSTEM_IS_WINDOWS]
	(__u8path(const Source&, char)): Add overloads for std::string and
	types convertible to std::string.
	(_Cvt::_S_wconvert): Add a new overload for char8_t strings and use
	codecvt_utf8_utf16 to do the correct conversion.

From-SVN: r278869