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

[Saleem Abdulrasool via cfe-commits]

compnerd wrote:

I'm not sure that this is a pattern we should adopt. @AaronBallman yes this is a compile time optimization only (LLVM's preference for forward declarations to avoid processing the header). I wonder if we can get away with `extern template` here for the benefit.

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