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

[Reid Kleckner via llvm-commits]

rnk wrote:

I haven't read past the title and I need to keep reading, but previously, VFS was a major compile-time culprit, so I like the idea of a dedicated fwd decl header for it. It's a big class with a wide interface that generates a lot of debug info, and any interface that touches the FS needs one of these now, so it's helpful to have a fwd decl to keep headers lean.

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