[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/include/SourceKit/Core/Context.h

@@ -35,7 +35,8 @@ class Context {
 public:
   Context(StringRef RuntimeLibPath,
           llvm::function_ref<
-              std::unique_ptr<LangSupport>(Context &)> LangSupportFactoryFn);
+              std::unique_ptr<LangSupport>(Context &)> LangSupportFactoryFn,
+          bool shouldDispatchNotificationsOnMain = true);
   ~Context();
 
   StringRef getRuntimeLibPath() const { return RuntimeLibPath; }

tools/SourceKit/include/SourceKit/Core/NotificationCenter.h

@@ -14,6 +14,7 @@
 #define LLVM_SOURCEKIT_CORE_NOTIFICATIONCENTER_H
 
 #include "SourceKit/Core/LLVM.h"
+#include "llvm/Support/Mutex.h"
 #include <functional>
 #include <vector>
 
@@ -23,9 +24,14 @@ typedef std::function<void(StringRef DocumentName)>
     DocumentUpdateNotificationReceiver;
 
 class NotificationCenter {
+  bool DispatchToMain;
   std::vector<DocumentUpdateNotificationReceiver> DocUpdReceivers;
+  mutable llvm::sys::Mutex Mtx;
 
 public:
+  explicit NotificationCenter(bool dispatchToMain);
+  ~NotificationCenter();
+
   void addDocumentUpdateNotificationReceiver(
       DocumentUpdateNotificationReceiver Receiver);