Fixes problem when calling llvm-ar from an unmodifiable directory.

This fixes a regression introduced by r185726: the new call to get
a unique file does not prepend the system temporary directory, so
we need to anchor on the file that the temporary file gets moved
to to ensure we're on the same file system.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@185825 91177308-0d34-0410-b5e6-96231b3b80d8