QArrayDataPointer: don't overload qSwap(), provide ADL-swap()

qSwap() is our wrapper around

   using std::swap;
   swap(lhs, rhs);

it needn't and shouldn't be overloaded.

ADL swap() should be, though, so qSwap(), std::ranges::swap() and all
the other adl_swap()s out there all find the optimized version.

Qt 5.15 has it correct, Qt 6 wrong. Fix it.

Can't pick to 6.2 because, while backwards-source-compatible, because
the generic qSwap() template provides the name for both qualified and
unqualified calls, it's not forwards-source-compatible: A new user of
ADL swap

   // compile error w/o `using std::swap`, pessimization otherwise:
   swap(dp1, dp2);

would break or performance-regress when going back to an older
version.

Pick-to: 6.3
Change-Id: I725949a4aa9ae438a182b4b7552ff2dced767e2f
Reviewed-by: Andrei Golubev <andrei.golubev@qt.io>
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>

src/corelib/tools/qarraydatapointer.h

@@ -415,7 +415,7 @@ public:
 };
 
 template <class T>
-inline void qSwap(QArrayDataPointer<T> &p1, QArrayDataPointer<T> &p2) noexcept
+inline void swap(QArrayDataPointer<T> &p1, QArrayDataPointer<T> &p2) noexcept
 {
     p1.swap(p2);
 }