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

[Adrian Prantl 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.
+
+Unlike other kinds of storage system like a file system, CAS is immutable. It
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adrian-prantl wrote:

```suggestion
Unlike other kinds of storage systems like file systems, CAS is immutable. It
```

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