Support: Have directory_iterator::status() return FindFirstFileEx/FindNextFile results on Windows.
Support: Have directory_iterator::status() return FindFirstFileEx/FindNextFile results on Windows.

This allows clients to avoid an unnecessary fs::status() call on each
directory entry. Because the information returned by FindFirstFileEx
is a subset of the information returned by a regular status() call,
I needed to extract a base class from file_status that contains only
that information.

On my machine, this reduces the time required to enumerate a ThinLTO
cache directory containing 520k files from almost 4 minutes to less
than 2 seconds.

Differential Revision: https://reviews.llvm.org/D38716

git-svn-id: ffe668792ed300d6c2daa1f6eba2e0aa28d7ec6c@315378 91177308-0d34-0410-b5e6-96231b3b80d8

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