[clang] [llvm] [mlir] [polly] [llvm] Forward declare VirtualFileSystem (PR #211761)

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> As @compnerd said, this is to avoid having to include the full header, which is fairly expensive. I am looking into using `extern template` for this instead of adding the new header, though.

Thanks! Hopefully `extern template` will suffice; I'm also pretty wary of following this pattern (I don't think it scales well) if we can avoid it.

https://github.com/llvm/llvm-project/pull/211761