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

[Jan Svoboda via llvm-commits]

jansvoboda11 wrote:

Have you considered switching the `vfs::FileSystem` hierarchy from `IntrusiveRefCntPtr` to `std::shared_ptr`? That would make the explicit specialization of the template unnecessary, and clients could just forward-declare `vfs::FileSystem` and include <shared_ptr> IIUC.

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