Add support for reading members out of thin archives.

Add support for reading members out of thin archives.

For now the Archive owns the buffers of the thin archive members.
This makes for a simple API, but all the buffers are destructed
only when the archive is destructed. This should be fine since we
close the files after mmap so we should not hit an open file
limit.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@242215 91177308-0d34-0410-b5e6-96231b3b80d8