QMovableArrayOps::Inserter cleanup [2/3]: mark most data members const

This makes explicit that the only thing that should be changing is
displaceFrom, which tracks the hole-filling process.

Requires rewriting of the dtor a bit, to skip the nInserts
modification.

Pick-to: 6.5
Change-Id: I1f0ce17cbeb171704f789624ad1be6ef70e58245
Reviewed-by: Ahmad Samir <a.samirh78@gmail.com>
(cherry picked from commit 75df3a2)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>
(cherry picked from commit e1d0213)

Author: marcmutz

src/corelib/tools/qarraydataops.h

@@ -661,11 +661,11 @@ struct QMovableArrayOps
 
     struct Inserter
     {
-        QArrayDataPointer<T> *data;
+        QArrayDataPointer<T> * const data;
         T *displaceFrom;
-        T *displaceTo;
-        qsizetype nInserts = 0;
-        size_t bytes;
+        T * const displaceTo;
+        const qsizetype nInserts = 0;
+        const size_t bytes;
 
         void verifyPost()
         { Q_ASSERT(displaceFrom == displaceTo); }
@@ -680,13 +680,14 @@ struct QMovableArrayOps
             ::memmove(static_cast<void *>(displaceTo), static_cast<void *>(displaceFrom), bytes);
         }
         ~Inserter() {
+            auto inserts = nInserts;
             if constexpr (!std::is_nothrow_copy_constructible_v<T>) {
                 if (displaceFrom != displaceTo) {
                     ::memmove(static_cast<void *>(displaceFrom), static_cast<void *>(displaceTo), bytes);
-                    nInserts -= qAbs(displaceFrom - displaceTo);
+                    inserts -= qAbs(displaceFrom - displaceTo);
                 }
             }
-            data->size += nInserts;
+            data->size += inserts;
         }
         Q_DISABLE_COPY(Inserter)