Add disk_space() to llvm::fs

Summary: Adapted from Boost::filesystem.
(This is a reapply by reverting commit r265062 and fixing the WinAPI part)

Differential Revision: http://reviews.llvm.org/D18467

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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@265068 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed