QUuid: fix qSwap() use in constexpr function

I don't know why the compilers didn't shout here, but what _should_
have happened is:

- qSwap<quint64>() gets instantiated

- the unqualified swap() call inside gets resolved to std::swap()

- std::swap() is not constexpr in C++17, so qSwap<quint64>() silently
  gets its constexpr dropped

- error, due to the use of non-constexpr function qSwap() in constexpr
  function bswap()

There's no way through the function that doesn't hit the qSwap(), so
that is also not the explanation. And, indeed, replacing qSwap() with
std::swap() gets me the expected error...

Before compilers get the idea, rewrite the code to not require
swapping.

Amends 686c02224c03735356bdab987bf62644eb34cc34.

Pick-to: 6.6
Change-Id: Ie1364bb2fd148bf995a8ffd321f77a6021176928
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/plugin/quuid.h

@@ -192,9 +192,10 @@ private:
     static constexpr Id128Bytes bswap(Id128Bytes b)
     {
         // 128-bit byte swap
-        b.data64[0] = qbswap(b.data64[0]);
-        b.data64[1] = qbswap(b.data64[1]);
-        qSwap(b.data64[0], b.data64[1]);
+        auto b0 = qbswap(b.data64[0]);
+        auto b1 = qbswap(b.data64[1]);
+        b.data64[0] = b1;
+        b.data64[1] = b0;
         return b;
     }
 };