Simplify two abstractions used in FileSpec::ForEachItemInDirectory, I'm seeing reports of crashes here and I can't quite figure out how we're getting them.  This won't fix the problem but it may make it a little easier to analyze the next crash report I get. <rdar://problem/28877808>
Simplify two abstractions used in FileSpec::ForEachItemInDirectory,
I'm seeing reports of crashes here and I can't quite figure out
how we're getting them.  This won't fix the problem but it may make
it a little easier to analyze the next crash report I get.
<rdar://problem/28877808>

(cherry picked from commit addb9d342cf8241c2f4dfd29d33984a24a9b93c1)

apple/swift-lldb,apple/swift-lldb,apple/swift-lldb,apple/swift-lldb,apple/swift-lldb,apple/swift-lldb