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

[Volodymyr Sapsai via cfe-commits]

vsapsai wrote:

The bigger idea is that not enabled stats should be negligibly cheap. As for me, the problem is in that not being true. Let me check how we have expensive stats all the time.

Extra fixes are good but I'm concerned they help only with these specific stats while others might still cause problems in different contexts.

https://github.com/llvm/llvm-project/pull/88427