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

[Paul Kirth via llvm-commits]

================
@@ -0,0 +1,195 @@
+//===- llvm/CAS/CASReference.h ----------------------------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_CAS_CASREFERENCE_H
+#define LLVM_CAS_CASREFERENCE_H
+
+#include "llvm/ADT/ArrayRef.h"
+#include "llvm/ADT/DenseMapInfo.h"
+#include "llvm/ADT/StringRef.h"
+
+namespace llvm {
+
+class raw_ostream;
+
+namespace cas {
+
+class ObjectStore;
+
+class ObjectHandle;
+class ObjectRef;
+
+/// Base class for references to things in \a ObjectStore.
+class ReferenceBase {
+protected:
+  struct DenseMapEmptyTag {};
+  struct DenseMapTombstoneTag {};
+  static constexpr uint64_t getDenseMapEmptyRef() { return -1ULL; }
+  static constexpr uint64_t getDenseMapTombstoneRef() { return -2ULL; }
+
+public:
+  /// Get an internal reference.
+  uint64_t getInternalRef(const ObjectStore &ExpectedCAS) const {
+#if LLVM_ENABLE_ABI_BREAKING_CHECKS
+    assert(CAS == &ExpectedCAS && "Extracting reference for the wrong CAS");
+#endif
+    return InternalRef;
+  }
+
+  unsigned getDenseMapHash() const {
+    return (unsigned)llvm::hash_value(InternalRef);
----------------
ilovepi wrote:

```suggestion
    return static_cast<unsigned>(llvm::hash_value(InternalRef));
```
I think we typically try to avoid C-style casting.

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