[SemaOverload] Use castAs<> instead of getAs<> to avoid dereference of nullptr

The pointer is always dereferenced inside BuildSimilarlyQualifiedPointerType, so assert the cast is correct instead of returning nullptr

clang/lib/Sema/SemaOverload.cpp

@@ -2405,9 +2405,8 @@ bool Sema::IsPointerConversion(Expr *From, QualType FromType, QualType ToType,
   if (FromType->isObjCObjectPointerType() && ToPointeeType->isVoidType() &&
       !getLangOpts().ObjCAutoRefCount) {
     ConvertedType = BuildSimilarlyQualifiedPointerType(
-                                      FromType->getAs<ObjCObjectPointerType>(),
-                                                       ToPointeeType,
-                                                       ToType, Context);
+        FromType->castAs<ObjCObjectPointerType>(), ToPointeeType, ToType,
+        Context);
     return true;
   }
   const PointerType *FromTypePtr = FromType->getAs<PointerType>();