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

[Fabrice de Gans via cfe-commits]

Steelskin wrote:

@vgvassilev @AaronBallman friendly ping
I would appreciate some guidance here. As mentioned in a previous comment, we have a few options here:
1. Keep the PR as-is and use the new include everywhere rather than forward-declare the class manually.
2. Reduce the scope by only adding the new include in `Instrumentor.h`, that is the only place where it is needed. The rest are merely replacements for the forward declaration.
3. Bite the bullet and include `VirtualFileSystem.h` in `Instrumentor.h`, but this will increase build times.

Note that if the guidance for `IntrusiveRefCntPtr` is incorrect, we should update this comment:
https://github.com/llvm/llvm-project/blob/697f0f3b20d371d9e417891d9cdfb6a3613d7828/llvm/include/llvm/ADT/IntrusiveRefCntPtr.h#L157

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