(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.