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

[Paul Kirth via llvm-commits]

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+# Content Addressable Storage
+
+## Introduction to CAS
+
+Content Addressable Storage, or `CAS`, is a storage system where it assigns
+unique addresses to the data stored. It is very useful for data deduplicaton
+and creating unique identifiers.
+
+Unlikely other kind of storage system like file system, CAS is immutable. It
+is more reliable to model a computation when representing the inputs and outputs
+of the computation using objects stored in CAS.
----------------
ilovepi wrote:

```suggestion
is more reliable to model a computation by representing the inputs and outputs
of the computation using objects stored in CAS.
```
I think this claim needs more support. If it's discussed more in a later section, then reference it and/or any other relevant citation.

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