[llvm-cov] Prevent llvm-cov from hanging when a symblink doesn't exist.

Summary:
Previous code hangs indefinitely when trying to iterate through a
symbol link file that points to an non-exist directory. This change
fixes the bug to make the addCollectedPath function exit ealier and
print out correct warning messages.

Patch by Yuke Liao (@liaoyuke).

Reviewers: Dor1s, vsk

Reviewed By: vsk

Subscribers: bruno, mgrang, llvm-commits

Differential Revision: https://reviews.llvm.org/D44960

llvm-svn: 329338