Editor: unify assist processor handling

Define the run type of the processor by its implementation instead of a
enum value of the provider. The execution of a processor inside the
assist now follows a unified procedure.

Change-Id: Ibe9fab324c6072e77702c2663946d7a9f562a085
Reviewed-by: <github-actions-qt-creator@cristianadam.eu>
Reviewed-by: Christian Stenger <christian.stenger@qt.io>
Reviewed-by: Christian Kandeler <christian.kandeler@qt.io>

src/plugins/clangcodemodel/clangdquickfixes.cpp

@@ -47,7 +47,7 @@ public:
     }
 
 private:
-    IAssistProposal *perform(const AssistInterface *interface) override
+    IAssistProposal *perform(AssistInterface *interface) override
     {
         m_interface = interface;