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

[Jan Svoboda via cfe-commits]

jansvoboda11 wrote:

> Not sure @jansvoboda11 perhaps if we want to cherry pick [b768a8c](https://github.com/llvm/llvm-project/commit/b768a8c1db85b9e84fd8b356570a3a8fbe37acf6) on `release/18.x`? Or should we land just a simple PR with just the function change above?

I can try pulling b768a8c1db85b9e84fd8b356570a3a8fbe37acf6 into the release branch.

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