tsan: use VReport instead of VPrintf in background thread

[dvyukov]

Summary

If there are multiple processes, it's hard to understand
what output comes from what process.
VReport prepends pid to the output. Use it.

Depends on D113982.

[dvyukov]

rebase

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

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");
+        VReport(1, "ThreadSanitizer: periodic memory flush\n");
         FlushShadowMemory();
         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",
+      VReport(1,
+              "ThreadSanitizer: memory flush check"
               (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");
+        VReport(1, "ThreadSanitizer: flushing memory due to RSS\n");
         FlushShadowMemory();
         rss = GetRSS();
         now = NanoTime();
-        VPrintf(1, "ThreadSanitizer: memory flushed RSS=%llu\n", (u64)rss>>20);
+        VReport(1, "ThreadSanitizer: memory flushed RSS=%llu\n",
+                (u64)rss >> 20);
       }
       last_rss = rss;
     }
 
     MemoryProfiler(now - start);
 
     // Flush symbolizer cache if requested.
     if (flags()->flush_symbolizer_ms > 0) {