[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
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adrian-prantl wrote:

```suggestion
Content Addressable Storage, or `CAS`, is a storage system that assigns
```

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