Main: Disable vptr checks for Singleton constructor (#2032)

UBSan complains a lot about the this pointer not being of type T* which is correct during construction, but ultimately doesn't matter. This change disables the vptr check on the Singleton constructor to avoid errors from UBSan.

OgreMain/include/OgreSingleton.h

@@ -80,6 +80,14 @@ template <typename T> class Singleton
     static T* msSingleton;
 
 public:
+#if defined(__has_attribute)
+#  if __has_attribute(no_sanitize)
+    // The `static_cast` happens so early in the construction of the inheriting
+    // classes that the `this` pointer is still detected as the super class
+    // pointer. Therefore, disabling vptr checks.
+    __attribute__((no_sanitize("vptr")))
+#  endif
+#endif
     Singleton(void)
     {
         OgreAssert(!msSingleton, "There can be only one singleton");