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

[Aaron Ballman via llvm-commits]

AaronBallman wrote:

> This allows for code that uses `IntrusiveRefCntPtr<vfs::FileSystem>` to compile when building LLVM as a dylib on Windows, without having to include the full `VirtualFileSystem.h` header.

Is there a reason that's a problem we need to solve? I assume including the full header is also viable, so is this just a build time optimization, or something else?

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