[patch] Add support to llvm-lib for producing thin archives

[Peter Collingbourne]

On Thu, Jul 16, 2015 at 03:33:15PM -0700, Rafael Espíndola wrote:
> This patch adds support for producing thin archives with llvm-lib.
> 
> I think the new lld should work with them out of the box. I am
> currently starting a windows build to test that out.

LGTM

Since we have started adding our own flags to this tool, it would be nice
to start a documentation page in docs/CommandGuide. We probably just need to
document our extensions and refer to the Microsoft documentation for the rest.

Thanks,
-- 
Peter