[clangd] JSONTracer: flush after writing event

[simark]

Summary

Let's say I use "CLANGD_TRACE=/tmp/clangd.json" and "tail -F
/tmp/clangd.json", I'll often see unfinished lines, where the rest of
the data is still in clangd's output buffer. Doing a flush in
rawEvent makes sure we can see all the data immediatly.

clangd/Trace.cpp

private:
   // Record an event. ph and pid are set.
   // Contents must be a list of the other JSON key/values.
   void rawEvent(StringRef Phase, json::Object &&Event) /*REQUIRES(Mu)*/ {
     // PID 0 represents the clangd process.
     Event["pid"] = 0;
     Event["ph"] = Phase;
     Out << Sep << formatv(JSONFormat, json::Value(std::move(Event)));
     Sep = ",\n";
+    Out.flush();
   }
 
   // If we haven't already, emit metadata describing this thread.
   void captureThreadMetadata() {
     uint64_t TID = get_threadid();
     std::lock_guard<std::mutex> Lock(Mu);
     if (ThreadsWithMD.insert(TID).second) {
       SmallString<32> Name;