[CAS] Add llvm-cas tools to inspect on-disk LLVMCAS

[cachemeifyoucan]

Add a command-line tool llvm-cas to inspect the OnDisk CAS for
debugging purpose. It can be used to lookup/update ObjectStore or
put/get cache entries from ActionCache, together with other debugging
capabilities.

[github-actions]

✅ With the latest revision this PR passed the C/C++ code formatter.

[github-actions]

✅ With the latest revision this PR passed the Python code formatter.

[ilovepi]

Largely I think this is in pretty good shape. If we're fine w/ the cl::opt structures vs. OptTable, then I think this is probably fine to land. I have a memory that we've been trying to migrate new tools to use OptTable over cl::opt structures for a few reasons, including the multicall binary support, which @petrhosek or @Prabhuk should be able to answer.

[petrhosek]

Largely I think this is in pretty good shape. If we're fine w/ the cl::opt structures vs. OptTable, then I think this is probably fine to land. I have a memory that we've been trying to migrate new tools to use OptTable over cl::opt structures for a few reasons, including the multicall binary support, which @petrhosek or @Prabhuk should be able to answer.

We're migrating all existing LLVM tools to OptTable and so ideally we would avoid introducing new tools that use cl::opt if possible, but if it's easier to land this PR as is and switch to OptTable in a follow up PR, that would be fine as well.

[cachemeifyoucan]

Oops, I accidentally merge this but didn't realize the merge base is wrong. Reopen: #166481