[clang] [llvm] [Support] Add VirtualOutputBackends to virtualize the output from tools (PR #68447)

[via llvm-commits]

https://github.com/zmodem commented:

I started reviewing the first commit, but wanted to ask some high-level questions which will make it easier to review:

The PR description doesn't say so, but the motivation for this is the CAS work, right?

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?

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