[llvm] Re-land #161264: [CAS] Add OnDiskDataAllocator (PR #162112)

[Steven Wu via llvm-commits]

cachemeifyoucan wrote:

Make sure I tested disabled configuration offline. And also fixes a GCC issue I see here: https://lab.llvm.org/buildbot/#/builders/140/builds/31936

https://github.com/llvm/llvm-project/pull/162112