fix: a clang warning [-Wshadow-field-in-constructor-modified] (#2780)

* Fix a clang warning from [-Wshadow-field-in-constructor-modified]

warning: modifying constructor parameter 'flag' that shadows a field of 'set_flag' [-Wshadow-field-in-constructor-modified]

* Change name based on review

include/pybind11/pybind11.h

@@ -1910,7 +1910,7 @@ template <return_value_policy Policy = return_value_policy::reference_internal,
 template <typename InputType, typename OutputType> void implicitly_convertible() {
     struct set_flag {
         bool &flag;
-        set_flag(bool &flag) : flag(flag) { flag = true; }
+        set_flag(bool &flag_) : flag(flag_) { flag_ = true; }
         ~set_flag() { flag = false; }
     };
     auto implicit_caster = [](PyObject *obj, PyTypeObject *type) -> PyObject * {