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

[Fabrice de Gans via cfe-commits]

Steelskin 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 a way to handle repetitive patterns we will grow a jungle of fwd.h files -- is that optimal?

@vgvassilev 
It's not so much the forward declaration that is moved to its header, it's the addition of the template instantiation for `IntrusiveRefCntPtrInfo<::llvm::vfs::FileSystem>`. Note that it is not required to use this header everywhere, I merely thought it was cleaner to do so. I can revert probably 90% of these headers if you'd prefer.

Note that this is what the `IntrusiveRefCntPtr` documentation recommends: https://github.com/llvm/llvm-project/blob/697f0f3b20d371d9e417891d9cdfb6a3613d7828/llvm/include/llvm/ADT/IntrusiveRefCntPtr.h#L157

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