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

[Steven Wu via llvm-commits]

cachemeifyoucan wrote:

> > Attempt a fix here: #164457
> 
> #164457 landed in https://lab.llvm.org/buildbot/#/builders/164/builds/14720 but the test (`CAS/./CASTests/4/22`) is still failing.

Attempt 2 here: https://github.com/llvm/llvm-project/pull/164493

I didn't figure out the real reason of failure last time. It was due to not all the fields in `std::array` is initialized.

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