Make main queue setup synchronous

Summary:
This can cause a deadlock if the main thread synchronously calls into some JS that creates a TurboModule. However, this is also the behaviour of legacy NativeModules. Furthermore, this also greatly simplifies how we'll fix async method invocation in TurboModules: T55255146.

Changelog:
[Internal]

Reviewed By: fkgozali

Differential Revision: D20364251

fbshipit-source-id: d0db85665506f08c51c2f33a123e69960923e7f3

ReactCommon/turbomodule/core/platform/ios/RCTTurboModuleManager.mm

@@ -17,6 +17,7 @@
 #import <React/RCTLog.h>
 #import <React/RCTModuleData.h>
 #import <React/RCTPerformanceLogger.h>
+#import <React/RCTUtils.h>
 #import <ReactCommon/TurboCxxModule.h>
 #import <ReactCommon/TurboModuleBinding.h>
 
@@ -326,7 +327,14 @@ static Class getFallbackClassFromName(const char *name)
 
   if ([[module class] respondsToSelector:@selector(requiresMainQueueSetup)] &&
       [[module class] requiresMainQueueSetup]) {
-    dispatch_async(dispatch_get_main_queue(), setupTurboModule);
+    /**
+     * If the main thread synchronously calls into JS that creates a TurboModule,
+     * we could deadlock. This behaviour is migrated over from the legacy NativeModule
+     * system.
+     *
+     * TODO(T63807674): Investigate the right migration plan off of this
+     */
+    RCTUnsafeExecuteOnMainQueueSync(setupTurboModule);
   } else {
     setupTurboModule();
   }