CompactContinuation: make sure what is being deleted by runObj

A lambda creates a struct with a function-call operator. So use of
'this' in a lambda body is always a bit ambiguous (the outer object?
Or the lambda itself? even though C++ defines it unabiguously to the
former).

Use a named capture variable instead of 'this' to make it obvious what
is being deleted.

Amends 699162c6fa6121cc496338f1d8d6e1b4287f7760.

Pick-to: 6.9
Change-Id: I48d82d788d495bac43c6adb114dfdd95a8dd0d1f
Reviewed-by: Ivan Solovev <ivan.solovev@qt.io>

src/corelib/thread/qfuture_impl.h

@@ -304,9 +304,9 @@ public:
         : Storage{std::forward<F>(func)}, promise(std::move(p)), parentFuture(f),
           threadPool(pool), type(Type::Async)
     {
-        runObj = QRunnable::create([this] {
-            this->runFunction();
-            delete this;
+        runObj = QRunnable::create([continuation = this] {
+            continuation->runFunction();
+            delete continuation;
         });
         runObj->setAutoDelete(false);
     }