Use error_mark_node after error in convert

While working on PR c/111903, I Noticed that
convert will convert integer_zero_node to that
type after an error instead of returning error_mark_node.
From what I can tell this was the old way of not having
error recovery since other places in this file does return
error_mark_node and the places I am replacing date from
when the file was imported into the repro (either via a gcc2 merge
or earlier).

I also had to update the objc front-end to allow for the error_mark_node
change, I suspect you could hit the ICE without this change though.

Bootstrapped and tested on x86_64-linux-gnu with no regressions.

gcc/ChangeLog:

	* convert.cc (convert_to_pointer_1): Return error_mark_node
	after an error.
	(convert_to_real_1): Likewise.
	(convert_to_integer_1): Likewise.
	(convert_to_complex_1): Likewise.

gcc/objc/ChangeLog:

	* objc-gnu-runtime-abi-01.cc (build_objc_method_call): Allow
	for error_operand after call to build_c_cast.
	* objc-next-runtime-abi-01.cc (build_objc_method_call): Likewise.
	* objc-next-runtime-abi-02.cc (build_v2_build_objc_method_call): Likewise.