llvm-ar: Let opening a directory failed in llvm-ar.

Linux cannot open directories with open(2), although cygwin and *bsd can.

Motivation: The test, Object/directory.ll, had been failing with --target=cygwin on Linux. XFAIL was improper for host issues.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@194257 91177308-0d34-0410-b5e6-96231b3b80d8