Add disk_space() to llvm::fs

Summary: Adapted from Boost::filesystem.

Reviewers: bruno, silvas

Subscribers: tberghammer, danalbert, llvm-commits, srhines

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

From: Mehdi Amini <mehdi.amini@apple.com>
llvm-svn: 265050