On Windows stat("foo/bar/../.") will resolve to "foo" even if that is a
non-directory and "foo/bar" does not exist. This is the expected
behaviour and consistent with boost::filesystem, so don't try to correct
it. The only unwanted behaviour is that stat("baz/") fails due to a
mingw bug (fixed in mingw-w64 v6.0.0) so add a workaround.

	PR libstdc++/88881
	* src/c++17/fs_ops.cc (canonical(const path&, error_code&))
	[_GLIBCXX_FILESYSTEM_IS_WINDOWS]: Normalize path, to match behaviour
	of filesystem::exists.
	(create_directories(const path&, error_code&)): Add assertions.
	(status(const path&, error_code&)) [_GLIBCXX_FILESYSTEM_IS_WINDOWS]:
	Add workaround for bug in _wstat for paths with trailing slash.
	* testsuite/27_io/filesystem/operations/create_directories.cc: Adjust
	for expected behaviour on mingw.
	* testsuite/experimental/filesystem/operations/create_directories.cc:
	Likewise.
	* testsuite/27_io/filesystem/operations/temp_directory_path.cc: Use
	"TMP" instead of "TMPDIR" and clean environment before each test. Do
	not test permissions on mingw targets.

Tested x86_64-linux and x86_64-w64-mingw32, committed to trunk.