Implement TrieRawHashMap which stores objects into a Trie based on the
hash of the object.

User needs to supply the hashing function and guarantees the uniqueness of
the hash for the objects to be inserted. Hash collision is not
supported

Add llvm::cas::ObjectStore abstraction and InMemoryCAS as a in-memory
CAS object store implementation.

The ObjectStore models its objects as:
* Content: An array of bytes for the data to be stored.
* Refs: An array of references to other objects in the ObjectStore.
And each CAS Object can be idenfied with an unqine ID/Hash.

ObjectStore supports following general action:
* Expected<ID> store(Content, ArrayRef<Ref>)
* Expected<Ref> get(ID)

It also introduces following types to interact with a CAS ObjectStore:
* CASID: Hash representation for an CAS Objects with its context to help
  print/compare CASIDs.
* ObjectRef: A light-weight ref for an object in the ObjectStore. It is
  implementation defined so it can be optimized for
  read/store/references depending on the implementation.
* ObjectHandle: A CAS internal light-weight handle to an loaded object in the
  ObjectStore. Underlying data for the object is guaranteed to be
  available and no error handling is required to access data. This is
  not exposed to the users of CAS from ObjectStore APIs.
* ObjectProxy: A proxy for the users of CAS to interact with the data
  inside CAS Object. It bundles a ObjectHandle and an ObjectStore
  instance.

Differential Revision: https://reviews.llvm.org/D133716

Add parameter to file lock API to allow exclusive file lock. Both Unix
and Windows support lock the file exclusively for write for one process
and LLVM OnDiskCAS uses exclusive file lock to coordinate CAS creation.

Add current downstream cas API and implementation that includes
OnDiskCAS implementation, different level of abstractions for CAS,
different utilities.

Allow loading external CAS implementation via PluginCAS. In this patch,
it adds:
* C APIs that can be implemented by plugin, from which LLVM can load the
  dylib to use external CAS implementation
* A PluginCAS, that implements vending external CAS implementation as
  llvm ObjectStore and ActionCache class.
* A libCASPluginTest dylib, that provides example external CAS
  implementation that wraps LLVM CAS for testing purpose.
* Add a unified way to load external CAS implementation.