Fix a deadlock in ModuleList when starting a standalone lldb client

Summary:
There was a deadlock was introduced by [PR llvm#146441](llvm#146441) which changed `CurrentThreadIsPrivateStateThread()` to `CurrentThreadPosesAsPrivateStateThread()`. This change caused the execution path in `ExecutionContextRef::SetTargetPtr()` to now enter a code block that was previously skipped, triggering `GetSelectedFrame()` which leads to a deadlock.

In particular,  one thread held `m_modules_mutex` and tried to acquire `m_language_runtimes_mutex` (via the notifier call chain), and another thread held `m_language_runtimes_mutex` and tried to acquire `m_modules_mutex` (via `ScanForGNUstepObjCLibraryCandidate`)

This fixes the deadlock by adding a scoped block around the mutex lock before the call to the notifier, and moved the notifier call outside of the mutex-guarded section.

Test Plan:
Tested manually

Author: qxy11

lldb/source/Core/ModuleList.cpp

@@ -215,30 +215,34 @@ ModuleList::~ModuleList() = default;
 
 void ModuleList::AppendImpl(const ModuleSP &module_sp, bool use_notifier) {
   if (module_sp) {
-    std::lock_guard<std::recursive_mutex> guard(m_modules_mutex);
-    // We are required to keep the first element of the Module List as the
-    // executable module.  So check here and if the first module is NOT an 
-    // but the new one is, we insert this module at the beginning, rather than 
-    // at the end.
-    // We don't need to do any of this if the list is empty:
-    if (m_modules.empty()) {
-      m_modules.push_back(module_sp);
-    } else {
-      // Since producing the ObjectFile may take some work, first check the 0th
-      // element, and only if that's NOT an executable look at the incoming
-      // ObjectFile.  That way in the normal case we only look at the element
-      // 0 ObjectFile. 
-      const bool elem_zero_is_executable 
-          = m_modules[0]->GetObjectFile()->GetType() 
-              == ObjectFile::Type::eTypeExecutable;
-      lldb_private::ObjectFile *obj = module_sp->GetObjectFile();
-      if (!elem_zero_is_executable && obj 
-          && obj->GetType() == ObjectFile::Type::eTypeExecutable) {
-        m_modules.insert(m_modules.begin(), module_sp);
-      } else {
+    {
+      std::lock_guard<std::recursive_mutex> guard(m_modules_mutex);
+      // We are required to keep the first element of the Module List as the
+      // executable module.  So check here and if the first module is NOT an
+      // but the new one is, we insert this module at the beginning, rather than
+      // at the end.
+      // We don't need to do any of this if the list is empty:
+      if (m_modules.empty()) {
         m_modules.push_back(module_sp);
+      } else {
+        // Since producing the ObjectFile may take some work, first check the
+        // 0th element, and only if that's NOT an executable look at the
+        // incoming ObjectFile.  That way in the normal case we only look at the
+        // element 0 ObjectFile.
+        const bool elem_zero_is_executable =
+            m_modules[0]->GetObjectFile()->GetType() ==
+            ObjectFile::Type::eTypeExecutable;
+        lldb_private::ObjectFile *obj = module_sp->GetObjectFile();
+        if (!elem_zero_is_executable && obj &&
+            obj->GetType() == ObjectFile::Type::eTypeExecutable) {
+          m_modules.insert(m_modules.begin(), module_sp);
+        } else {
+          m_modules.push_back(module_sp);
+        }
       }
     }
+    // Release the mutex before calling the notifier to avoid deadlock
+    // NotifyModuleAdded should be thread-safe
     if (use_notifier && m_notifier)
       m_notifier->NotifyModuleAdded(*this, module_sp);
   }