AsyncTask: Centralize setting global future synchronizer

Move the global FutureSynchronizer to Utils and use it
by default for AsyncTask (if that is in the main thread).
This way setting a synchronizer is less prone to be forgotten.
Individual uses can still override this.

Change-Id: I1a12bf4d7f4cb5be99668bf0a4797108c2fe6448
Reviewed-by: <github-actions-qt-creator@cristianadam.eu>
Reviewed-by: Jarek Kobus <jaroslaw.kobus@qt.io>

src/plugins/clangcodemodel/clangdlocatorfilters.cpp

@@ -10,8 +10,6 @@
 #include <cppeditor/cppeditortr.h>
 #include <cppeditor/cpplocatorfilter.h>
 
-#include <extensionsystem/pluginmanager.h>
-
 #include <languageclient/currentdocumentsymbolsrequest.h>
 #include <languageclient/locatorfilter.h>
 
@@ -186,7 +184,6 @@ static LocatorMatcherTask currentDocumentMatcher()
     };
 
     const auto onFilterSetup = [=](Async<void> &async) {
-        async.setFutureSynchronizer(ExtensionSystem::PluginManager::futureSynchronizer());
         async.setConcurrentCallData(filterCurrentResults, *storage, *resultStorage,
                                     TextDocument::currentTextDocument()->plainText());
     };