[llvm] Support: Add proxies for raw_ostream and raw_pwrite_stream (PR #113362)

[Steven Wu via llvm-commits]

cachemeifyoucan wrote:

Migrated from https://github.com/llvm/llvm-project/pull/68447

This is for potential future CAS support. For @zmodem 's question from preview PR
> LLVM already has a "virtual filesystem", and I thought at least for Clang most file operations go through that already. So what gaps are there that need to be filled for the needs of CAS?

VFS is for the input to the compilation and VOB (VirtualOutputBackends) are for the compilation outputs. This allows tools to write into virtual outputs that can be direct to file system, memory, CAS, or forked into multiple locations.

https://github.com/llvm/llvm-project/pull/113362