[llvm] [CAS] LLVMCAS implementation (PR #68448)

[David Blaikie via llvm-commits]

dwblaikie wrote:

(be great if this could be broken up a bit more - like the TrieRawHashMap looks like it'd be a fine standalone patch to introduce the data structure and its unit tests - maybe similarly with the other VirtualOutputBackends patch, the raw_ostream_proxy might be separable (& it'd be nice if the other parts of the functionality were built up more incrementally too))

But maybe it's just not practical - it's unfortunate when things get to this state, though I understand the value of prototyping out of tree - it makes it hard to adequately review things when they come into the project this way.

https://github.com/llvm/llvm-project/pull/68448