Introduce and use FileName::exists()

This can use the faster route through QFileInfo::exist now.

Change-Id: Idb41b5d5185d7f02eacba498fb01f483d95e8d57
Reviewed-by: Eike Ziller <eike.ziller@theqtcompany.com>
Reviewed-by: Christian Stenger <christian.stenger@digia.com>

src/plugins/debugger/debuggeritemmanager.cpp

@@ -298,7 +298,7 @@ void DebuggerItemManager::readLegacyDebuggers(const FileName &file)
         if (fn == QLatin1String("auto"))
             continue;
         FileName command = FileName::fromUserInput(fn);
-        if (!command.toFileInfo().exists())
+        if (!command.exists())
             continue;
         if (findByCommand(command))
             continue;