[clang] [llvm] [clang][deps] Overload `Filesystem::exists` in `DependencyScanningFilesystem` to have it use cached `status` (PR #88152)

[Alexandre Ganea via cfe-commits]

aganea wrote:

This patch doesn`t improve my usage, it seems I'm hitting a different codepath than you do. I'll investigate.

https://github.com/llvm/llvm-project/pull/88152