Fix bugs in create_directory implementation.

Libc++ was incorrectly reporting an error when the target of create_directory
already exists, but was not a directory. This behavior is not specified
in the most recent standard, which says no error should be reported.

Additionally, libc++ failed to report an error when the attribute directory
path didn't exist or didn't name a directory. This has been fixed as well.

Although it's not clear if we should call status or symlink_status on the
attribute directory. This patch chooses to still call status.

llvm-svn: 337888