[SourceKit] If diagnostics are 'stale' for a particular snapshot then ignore them and only return the syntactic parser diagnostics (#10388)

This makes sure that diagnostics returned for a particular state of source buffer are consistent and accurate.
rdar://32769873

tools/SourceKit/lib/Core/NotificationCenter.cpp

@@ -15,20 +15,33 @@
 
 using namespace SourceKit;
 
+NotificationCenter::NotificationCenter(bool dispatchToMain)
+  : DispatchToMain(dispatchToMain) {
+}
+NotificationCenter::~NotificationCenter() {}
+
 void NotificationCenter::addDocumentUpdateNotificationReceiver(
     DocumentUpdateNotificationReceiver Receiver) {
 
-  WorkQueue::dispatchOnMain([this, Receiver]{
-    DocUpdReceivers.push_back(Receiver);
-  });
+  llvm::sys::ScopedLock L(Mtx);
+  DocUpdReceivers.push_back(Receiver);
 }
 
 void NotificationCenter::postDocumentUpdateNotification(
     StringRef DocumentName) const {
-  
-  std::string DocName = DocumentName;
-  WorkQueue::dispatchOnMain([this, DocName]{
-    for (auto &Fn : DocUpdReceivers)
-      Fn(DocName);
-  });
+
+  std::vector<DocumentUpdateNotificationReceiver> recvs;
+  {
+    llvm::sys::ScopedLock L(Mtx);
+    recvs = DocUpdReceivers;
+  }  
+  std::string docName = DocumentName;
+  auto sendNote = [recvs, docName]{
+    for (auto &Fn : recvs)
+      Fn(docName);
+  };
+  if (DispatchToMain)
+    WorkQueue::dispatchOnMain(sendNote);
+  else
+    sendNote();
 }