A Derived CFunEqCan does not "own" its FlatMetaTv (fmv), and should not
update it.  But one caller (canCFunEqCan) was failing to satisfy the
precondition to dischargeFmv, which led to a crash (Trac #15170).

I fixed this by making dischargeFmv handle Deriveds (to avoid forcing
each caller to do so separately).

NB: this does not completely fix the original #15170 bug, but I'll
explain that on the ticket.  The test case for this patch is actually
the program in comment:1.