[CAS] Add OnDiskGraphDB and OnDiskKeyValueDB (#114102)

Add OnDiskGraphDB and OnDiskKeyValueDB that can be used to implement
ObjectStore and ActionCache respectively. Those are on-disk persistent
storage that build upon OnDiskTrieHashMap and implements key functions
that are required by LLVMCAS interfaces.

This abstraction layer defines how the objects are hashed and stored on
disk. OnDiskKeyValueDB is a basic OnDiskTrieHashMap while OnDiskGraphDB
also defines:
* How objects of various size are store on disk and are referenced by
  the trie nodes.
* How to store the references from one stored object to another object
  that is referenced.

In addition to basic APIs for ObjectStore and ActionCache, other
advances database configuration features can be implemented in this
layer without exposing to the users of the LLVMCAS interface. For
example, OnDiskGraphDB has a faulty in function to fetch data from an
upstream OnDiskGraphDB if the data is missing.

Author: cachemeifyoucan

llvm/include/llvm/CAS/OnDiskDataAllocator.h

@@ -64,7 +64,7 @@ class OnDiskDataAllocator {
 
   /// \returns the buffer that was allocated at \p create time, with size
   /// \p UserHeaderSize.
-  MutableArrayRef<uint8_t> getUserHeader();
+  MutableArrayRef<uint8_t> getUserHeader() const;
 
   size_t size() const;
   size_t capacity() const;