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

[Alexandre Ganea via cfe-commits]

aganea wrote:

> In the meantime, are you able to work around this for your non-modular use-case by applying your change when building your own compiler downstream?

Yes I fixed it in our downstream. I am more worried about other users, since the initial version that @hyp committed a while ago in e1f4c4aad27992d6b8a0b8d85af42c14fa68c298 didn't suffer of these issues.

> Also, just to be sure, this is not a regression, correct?

Not per se, not from the latest commits. It seems the issue was introduced when module support was added in 9ab6d8236b176bf9dd43741f4d874a8afebed99c. Unfortunately there are users like us with big C++ codebases and stuck with regular #includes, no modules for now.

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