[NFC][TSan] Make Context::nreported atomic (#228602)

Make Context::nreported an atomic_uint32_t so that Finalize and
__tsan_report_count can read it without relying on report_mtx.

Assisted-by: Gemini
GitOrigin-RevId: 8143db6025ae2092ca90f8b0efc43918a94df69f
4 files changed