[clang][deps] Overload `Filesystem::exists` in `DependencyScanningFilesystem`...

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

As-is, calls to `exists()` fallback on the implementation in
`ProxyFileSystem::exists` which explicitly calls out to the underlying
`FS`, which for the `DependencyScanningFilesystem` (overlay) is the real
underlying filesystem.

Instead, directly overloading `exists` allows us to have it rely on the
cached `status` behavior used elsewhere by the
`DependencyScanningFilesystem`.