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

[Jan Svoboda via cfe-commits]

jansvoboda11 wrote:

I'd like to see a unit test specific to `DependencyScanningFilesystem`, similar to what I have here: https://github.com/llvm/llvm-project/pull/68645.

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