De-inline QGenericRunnable overrides

Pins the vtable to a single TU and therefore prevents duplicate
vtables, -Wweak-vtables warnings and false-positive dynamic_casts.

This requires exporting QGenericRunnable, which hopefully won't
also export the nested class.

Task-number: QTBUG-45582
Change-Id: Ie1f29d25b0dcdee7654c33c497e8e0350d12e311
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
(cherry picked from commit be1b589cb9f147762a5a73d2b3a44778b7ae7502)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/corelib/thread/qrunnable.cpp

@@ -21,6 +21,17 @@ QRunnable *QRunnable::warnNullCallable()
     return nullptr;
 }
 
+
+QRunnable::QGenericRunnable::~QGenericRunnable()
+{
+    runHelper->destroy();
+}
+
+void QRunnable::QGenericRunnable::run()
+{
+    runHelper->run();
+}
+
 /*!
     \class QRunnable
     \inmodule QtCore

src/corelib/thread/qrunnable.h

@@ -42,7 +42,7 @@ private:
     class QGenericRunnable;
 };
 
-class QRunnable::QGenericRunnable : public QRunnable
+class Q_CORE_EXPORT QRunnable::QGenericRunnable : public QRunnable
 {
     // Type erasure, to only instantiate a non-virtual class per Callable:
     class HelperBase
@@ -91,14 +91,9 @@ public:
         : runHelper(new Helper<std::decay_t<Callable>>(std::forward<Callable>(c)))
     {
     }
-    ~QGenericRunnable() override
+    ~QGenericRunnable() override;
-    {
+
-        runHelper->destroy();
+    void run() override;
-    }
-    void run() override
-    {
-        runHelper->run();
-    }
 };
 
 namespace QtPrivate {