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

[Alexandre Ganea via cfe-commits]

aganea wrote:

> As an alternative approach: could we turn these into member variables, make them non-atomic and take care to update the stats of the superior `FileManager` whenever an inferior `FileManager` goes out of scope? (e.g. after implicitly building a module)

As suggested.

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