N4100 states that an error shall be reported if
`!exists(p) || !is_directory(p)`. We were missing the first half of the
conditional. Invert the error and normal code paths to make the code
easier to follow.
git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@294127 91177308-0d34-0410-b5e6-96231b3b80d8