ARM-Darwin: re-enable -momit-leaf-frame-pointer.

In r279546 I disabled all frame pointer elimination at the front-end on
ARM-Darwin (and warned about it) because before that the backend had been
silently ignoring these options. It turns out we didn't ignore
-momit-leaf-frame-pointer though, just the more general -fomit-frame-pointer.

So this re-enables passing that down to CodeGen so that everything really does
continue working as before (with better diagnostics).

llvm-svn: 293311