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

[Vassil Vassilev via cfe-commits]

vgvassilev wrote:

@compnerd, @Steelskin: This uncovers an annoying limitation where repeating forward declarations will need to be annotated and there is no easy way how to centralize them in a single place because of the different usage patterns. If we merge this as the as a way to handle repetitive patterns we will grow a jungle of fwd.h files -- is that optimal?

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