libstdc++: Ensure root-dir converted to forward slash (PR93244)

	PR libstdc++/93244
	* include/bits/fs_path.h (path::generic_string<C,A>)
	[_GLIBCXX_FILESYSTEM_IS_WINDOWS]: Convert root-dir to forward-slash.
	* testsuite/27_io/filesystem/path/generic/generic_string.cc: Check
	root-dir is converted to forward slash in generic pathname.
	* testsuite/27_io/filesystem/path/generic/utf.cc: New test.
	* testsuite/27_io/filesystem/path/generic/wchar_t.cc: New test.