Use newDFunName for both manual and derived instances (#17339)

Issue #17339 (closed) was caused by using a slightly different version of newDFunName for derived instances that, confusingly enough, did not take all arguments to the class into account when generating the DFun name. I cannot think of any good reason for doing this, so this patch uses newDFunName uniformly for both derived instances and manually written instances alike.

Fixes #17339 (closed).