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

[Vassil Vassilev via cfe-commits]

vgvassilev wrote:

For the sake of progress, I’d summarize my understand: it seems that the overall guidance is leaning towards having this forward declarations in the header file as initially proposed with some constrains which I believe were discussed and addressed. 

Please rebase the PR. 

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