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

[Artem Chikin via llvm-commits]

artemcm wrote:

> I'd like to see a unit test specific to `DependencyScanningFilesystem`, similar to what I have here: #68645.

Done! Added one in `DependencyScanningFilesystemTest.cpp`. Thank you.

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