[llvm] Support: Add vfs::OutputBackend and OutputFile to virtualize compiler outputs (PR #113363)

[Steven Wu via llvm-commits]

cachemeifyoucan wrote:

Looks like GCC ICE on some `constexpr`. Attempted a quick fix here: https://github.com/llvm/llvm-project/commit/fb60defd839d2eac7428abe60b362145a6888e69

If that doesn't work, I will revert.

https://github.com/llvm/llvm-project/pull/113363