Clangd: Create correct code assist processor in provider

... for the context of the code assist instead of using a callback from
the text editor.

Change-Id: I636b1b267a944895d4ff732e3c0a8811332ec14f
Reviewed-by: Christian Kandeler <christian.kandeler@qt.io>

src/plugins/clangcodemodel/clangdclient.h

@@ -98,6 +98,7 @@ private:
     class FollowSymbolData;
     class VirtualFunctionAssistProcessor;
     class VirtualFunctionAssistProvider;
+    class ClangdCompletionAssistProvider;
     Private * const d;
 };