[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/Context.cpp

@@ -18,8 +18,9 @@ using namespace SourceKit;
 
 SourceKit::Context::Context(StringRef RuntimeLibPath,
     llvm::function_ref<std::unique_ptr<LangSupport>(Context &)>
-    LangSupportFactoryFn) : RuntimeLibPath(RuntimeLibPath),
-    NotificationCtr(new NotificationCenter()) {
+    LangSupportFactoryFn,
+    bool shouldDispatchNotificationsOnMain) : RuntimeLibPath(RuntimeLibPath),
+    NotificationCtr(new NotificationCenter(shouldDispatchNotificationsOnMain)) {
   // Should be called last after everything is initialized.
   SwiftLang = LangSupportFactoryFn(*this);
 }