[llvm] [CAS] Add UnifiedOnDiskCache and OnDiskCAS (PR #114103)

[Justin Bogner via llvm-commits]

================
@@ -0,0 +1,612 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+//
+/// \file
+/// Encapsulates \p OnDiskGraphDB and \p OnDiskKeyValueDB instances within one
+/// directory while also restricting storage growth with a scheme of chaining
+/// the two most recent directories (primary & upstream), where the primary
+/// "faults-in" data from the upstream one. When the primary (most recent)
+/// directory exceeds its intended limit a new empty directory becomes the
+/// primary one.
+///
+/// Within the top-level directory (the path that \p UnifiedOnDiskCache::open
+/// receives) there are directories named like this:
+///
+/// 'v<version>.<x>'
+/// 'v<version>.<x+1'
----------------
bogner wrote:

```suggestion
/// 'v<version>.<x+1>'
```

https://github.com/llvm/llvm-project/pull/114103