tsan: better maintain current time in the background thread

[dvyukov]

Summary

Update now after long operations so that we don't use
stale value in subsequent computations.

Depends on D113981.

[dvyukov]

rebase

compiler-rt/lib/tsan/rtl/tsan_rtl.cpp

static void *BackgroundThread(void *arg) {
   // This is a non-initialized non-user thread, nothing to see here.
   // We don't use ScopedIgnoreInterceptors, because we want ignores to be
   // enabled even when the thread function exits (e.g. during pthread thread
   // shutdown code).
   cur_thread_init()->ignore_interceptors++;
   const u64 kMs2Ns = 1000 * 1000;
   const u64 start = NanoTime();
 
-  u64 last_flush = NanoTime();
+  u64 last_flush = start;
   uptr last_rss = 0;
   for (int i = 0;
       atomic_load(&ctx->stop_background_thread, memory_order_relaxed) == 0;
       i++) {
     SleepForMillis(100);
     u64 now = NanoTime();
 
     // Flush memory if requested.
     if (flags()->flush_memory_ms > 0) {
       if (last_flush + flags()->flush_memory_ms * kMs2Ns < now) {
         VPrintf(1, "ThreadSanitizer: periodic memory flush\n");
         FlushShadowMemory();
-        last_flush = NanoTime();
+        now = last_flush = NanoTime();
       }
     }
     if (flags()->memory_limit_mb > 0) {
       uptr rss = GetRSS();
       uptr limit = uptr(flags()->memory_limit_mb) << 20;
       VPrintf(1, "ThreadSanitizer: memory flush check"
                  " RSS=%llu LAST=%llu LIMIT=%llu\n",
               (u64)rss >> 20, (u64)last_rss >> 20, (u64)limit >> 20);
       if (2 * rss > limit + last_rss) {
         VPrintf(1, "ThreadSanitizer: flushing memory due to RSS\n");
         FlushShadowMemory();
         rss = GetRSS();
+        now = NanoTime();
         VPrintf(1, "ThreadSanitizer: memory flushed RSS=%llu\n", (u64)rss>>20);
       }
       last_rss = rss;
     }
 
     MemoryProfiler(now - start);
 
     // Flush symbolizer cache if requested.