Implement the proposed semantics from P1164R0, which reverts the changes
of LWG 2935. This means that failure to create a directory because a
non-directory already exists with that name will be reported as an
error.

While rewriting the function, also fix PR 87846, which is a result of
the C++17 changes to how a trailing slash on a path affects the last
component of a path.

	PR libstdc++/86910
	PR libstdc++/87846
	* src/filesystem/ops.cc (experimental::create_directories): Report
	an error when the path resolves to an existing non-directory (P1164).
	* src/filesystem/std-ops.cc (create_directories): Likewise. Handle
	empty filenames due to trailing slashes.
	* testsuite/27_io/filesystem/operations/create_directories.cc: Test
	when some component of the path exists and is not a directory. Test
	trailing slashes.
	* testsuite/experimental/filesystem/operations/create_directories.cc:
	Likewise.

From-SVN: r266598