Implement sys::fs::copy_file using the macOS copyfile(3) API

Implement sys::fs::copy_file using the macOS copyfile(3) API
to support APFS clones.

This patch adds a Darwin-specific implementation of
llvm::sys::fs::copy_file() that uses the macOS copyfile(3) API to
support APFS copy-on-write clones, which should be faster and much
more space efficient.

https://developer.apple.com/library/archive/documentation/FileManagement/Conceptual/APFS_Guide/ToolsandAPIs/ToolsandAPIs.html

Differential Revision: https://reviews.llvm.org/D60802

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@358628 91177308-0d34-0410-b5e6-96231b3b80d8