[SYCL] Clear plugins after unload (#6846)

Unloading plugins was recently separated from the cleanup phase to allow
the mock plugin ensure that the mock plugin is the lone plugin. However,
after unloading the plugins vector is not currenly cleared. This commit
clears the plugins vector after plugins have been unloaded.

Signed-off-by: Larsen, Steffen <[email protected]>

Author: steffenlarsen

sycl/source/detail/global_handler.cpp

@@ -132,6 +132,8 @@ void GlobalHandler::unloadPlugins() {
       Plugin.unload();
     }
   }
+  // Clear after unload to avoid uses after unload.
+  GlobalHandler::instance().getPlugins().clear();
 }
 
 void shutdown() {