[VFS] Add support for "no_push" to VFS recursive iterators.

The "regular" file system has a useful feature that makes it possible to
stop recursing when using the recursive directory iterators. This
functionality was missing for the VFS recursive iterator and this patch
adds that.

Differential revision: https://reviews.llvm.org/D53465

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@345793 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed