Use const ref instead of value for Twine in the disk_space() API

Thanks Rui for noticing!

From: Mehdi Amini <mehdi.amini@apple.com>

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@265074 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed