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

[Jan Svoboda via cfe-commits]

jansvoboda11 wrote:

Clang test now looks good to me. Might be nice to drop `InstrumentingInMemoryFilesystem` in favor of the existing `InstrumentingFilesystem` (that I added just moments ago, sorry!) wrapped around a normal `InMemoryFileSystem`, but I'm happy to do that myself in a follow-up commit.

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