[llvm] [CAS] Add OnDiskGraphDB and OnDiskKeyValueDB (PR #114102)

[Steven Wu via llvm-commits]

cachemeifyoucan wrote:

I can't see any obvious logic mistakes in the code. I haven't dig too deep but it seems `DataRecordHandle::getRefsRelOffset()` is mis-compiled if `LayoutFlag::getLayoutFlag()` and its `unpack` function is inlined. 

https://github.com/llvm/llvm-project/pull/114102