[ThinLTO/gold] Avoid race with cache pruner by copying to temp files

Summary:
This will avoid the race condition described in the review for D37993.

I believe that the Path parameter to AddBufferFn is no longer utilized.
I would prefer to remove that as a follow up clean up patch to reduce
the diffs in this patch.

Reviewers: pcc

Reviewed By: pcc

Subscribers: inglorion, llvm-commits

Differential Revision: https://reviews.llvm.org/D43396

llvm-svn: 325618