[clang-scan-deps] Fix contention when updating `TrackingStatistic`s in hot...

[clang-scan-deps] Fix contention when updating `TrackingStatistic`s in hot code paths in `FileManager`. (#88427)

`FileManager::getDirectoryRef()` and `FileManager::getFileRef()` are hot code paths in `clang-scan-deps`. These functions are updating on every call a few atomics related to printing statistics, which causes contention on high core count machines.

![Screenshot 2024-04-10
214123](https://github.com/llvm/llvm-project/assets/37383324/5756b1bc-cab5-4612-8769-ee7e03a66479)

![Screenshot 2024-04-10
214246](https://github.com/llvm/llvm-project/assets/37383324/3d560e89-61c7-4fb9-9330-f9e660e8fc8b)

![Screenshot 2024-04-10
214315](https://github.com/llvm/llvm-project/assets/37383324/006341fc-49d4-4720-a348-7af435c21b17)

After this patch we make the variables local to the `FileManager`.

In our test case, this saves about 49 sec over 1 min 47 sec of `clang-scan-deps` run time (1 min 47 sec before, 58 sec after). These figures are after applying my suggestion in https://github.com/llvm/llvm-project/pull/88152#issuecomment-2049803229, that is:
```
static bool shouldCacheStatFailures(StringRef Filename) {
  return true;
}
```
Without the above, there's just too much OS noise from the high volume of `status()` calls with regular non-modules C++ code. Tested on Windows with clang-cl.