[Support] Listing a directory containing dangling symlinks is not an error.

Summary:
Reporting this as an error required stat()ing every file, as well as seeming
semantically questionable.

Reviewers: vsk, bkramer

Subscribers: mgrang, kristina, llvm-commits, liaoyuke

Differential Revision: https://reviews.llvm.org/D52648

llvm-svn: 343460