[llvm] [CAS] Add LLVMCAS library with InMemoryCAS implementation (PR #114096)

[Paul Kirth via llvm-commits]

https://github.com/ilovepi commented:

This is a large patch, so I haven't read all the C++ closely enough to say anything about it. Mostly, I've left comments on the documentation, and pointed out a few things I saw while skimming the implementation.

I think overall a bit more explanation about how some of the key classes interoperate may be warranted. For instance I really don't understand why some of the reinterpret casts should be legal in `InMemroyInlineObject`. 

The other thing I see is a pattern of `foo(){return fooImpl();}`, but `fooImpl()` isn't private or (from what I can see) an extension point. I'm not sure I understand the design choice here, since I don't recall seeing it used this way much in LLVM. Normally the Impls are either private, virtual, or in some other module/namespace. It's a big project, so maybe I've just missed something though.

I'll try to take a more detailed look at the implementation code, but I'd encourage others to look too, since this is a very large, complicated patch set.

https://github.com/llvm/llvm-project/pull/114096