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

[Alexandre Ganea via llvm-commits]

aganea 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 [b768a8c](https://github.com/llvm/llvm-project/commit/b768a8c1db85b9e84fd8b356570a3a8fbe37acf6) into the release branch.

Sorry I've been misleading, pushing b768a8c on 18.x doesn't fix the issue. The problem is around the directory queries and the function in b768a8c returns `false` for that case (thus no caching of errors).

Running with:
```
static bool shouldCacheStatFailures(StringRef Filename) {
  StringRef Ext = llvm::sys::path::extension(Filename);
  if (Ext.empty())
    return false; // This may be the module cache directory.
  return true;
}
```
Takes 4 min 8 sec.

Then running with:
```
static bool shouldCacheStatFailures(StringRef Filename) {
  return true;
}
```
Takes 1 min 47 sec.

I think something more involved would be needed here. @jansvoboda11 in which case we don't want to consider the file system immutable during the execution of clang-scan-deps?

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