Fixes errors with FS iterators caused by https://reviews.llvm.org/D44960

Summary:
In https://reviews.llvm.org/D44960, file status check is executed every
time a real file system directory iterator is constructed or
incremented, and emits an error code. This change list fixes the errors
in VirtualFileSystem caused by https://reviews.llvm.org/D44960.

Patch by Yuke Liao (@liaoyuke).

Reviewers: vsk, pcc, zturner, liaoyuke

Reviewed By: vsk

Subscribers: mgrang, cfe-commits

Differential Revision: https://reviews.llvm.org/D45178

llvm-svn: 329223