r191394 - Produce an error for unknown -f options.

[Rafael Espíndola]

> This can be done with a joined option:
> def fbuiltin_ : Joined<["-"], "fbuiltin-">;
> def fno_builtin_ : Joined<["-"], "fno-builtin-">;
>
> Which Nick says is how we would actually implement it now that LLVM has TLI.

I checked gcc's behavior:

* There is only -fbuiltin, not -fbuiltin-FUNCTION
* There is both -fno-builtin and -fno-builtin-FUNCTION and gcc eats
any function name.

I would be nice to be stricter than gcc in here, but I will implement
gcc's behavior first.

Cheers,
Rafael