Reapply [VFS] Ignore broken symlinks in the directory iterator.

Modified the tests to accept any iteration order, to run only on Unix, and added
additional error reporting to investigate SystemZ bot issue.

The VFS directory iterator and recursive directory iterator behave differently
from the LLVM counterparts. Once the VFS iterators hit a broken symlink they
immediately abort. The LLVM counterparts don't stat entries unless they have to
descend into the next directory, which allows to recover from this issue by
clearing the error code and skipping to the next entry.

This change adds similar behavior to the VFS iterators. There should be no
change in current behavior in the current CLANG source base, because all
clients have loop exit conditions that also check the error code.

This fixes rdar://problem/30934619.

Differential Revision: https://reviews.llvm.org/D30768

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@297693 91177308-0d34-0410-b5e6-96231b3b80d8