[lld/mac] Abort link immediately on invalid -undefined argument

I tried `-undefined suppress` without `-flat_namespace`.
lld printed `'-undefined suppress' only valid with '-flat_namespace'`
followed by many many screenfuls of error messages about undefined
symbols, making the original diag hard to see.

This is probably the common case when using `-undefined`, so let's
just abort the link immediately when there's an invalid `-undefined`
arg.

Differential Revision: https://reviews.llvm.org/D139559