(comptypes, convert_for_assignment): Check for Objective-C protocols (non-reflexive use of maybe_objc_comptypes).

(comptypes, convert_for_assignment): Check for
	Objective-C protocols (non-reflexive use of maybe_objc_comptypes).
	(comp_target_types): Rewrite so that maybe_objc_comptypes checks
	protocols when processing an Objective-C module.  For C modules,
	the code path is unchanged.
	(convert_for_assignment): Improve the error message for
	incompatible message arguments.

From-SVN: r4067