Fix MSVC warnings

* Missing `this` captures
* Implicit size_t -> int conversion
* Unused argument
* Suppress warnings in clang headers

Change-Id: I7083ce6ab22ee22ecc1258539e77c790acc78df1
Reviewed-by: hjk <hjk@qt.io>

src/plugins/clangformat/clangformatplugin.cpp

@@ -79,6 +79,7 @@ public:
     TextEditor::CodeStyleEditorWidget *createCodeStyleEditor(
         TextEditor::ICodeStylePreferences *preferences, QWidget *parent = nullptr) override
     {
+        Q_UNUSED(preferences);
         if (!parent)
             return new ClangFormatConfigWidget;
         return new ClangFormatConfigWidget(SessionManager::startupProject());