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

[Steven Wu via llvm-commits]

cachemeifyoucan wrote:

@zmodem Sorry I totally missed you comments. I thought no one was reviewing so I kind of abandoned the process. 

Recreate the PR in a stack for new reviews. All of your comments should be addressed:
```
3713985 Support: Add proxies for raw_ostream and raw_pwrite_stream
  ✨  Created new Pull Request #113362: https://github.com/llvm/llvm-project/pull/113362
3587264 Support: Add vfs::OutputBackend and OutputFile to virtualize compiler outputs
  ✨  Created new Pull Request #113363: https://github.com/llvm/llvm-project/pull/113363
c59aacd Frontend: Adopt llvm::vfs::OutputBackend in CompilerInstance
  ✨  Created new Pull Request #113364: https://github.com/llvm/llvm-project/pull/113364
```

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