c: some cleanups for the comptypes hierarchy of functions

comptypes returns an int where a value of two indicates that a warning
is needed due to a mismatch of attributes.  Change all related functions
to return a boolean and use comptypes_internal in the one caller
(comp_target_types) that requires the additional information.  This allows
simplifying the code a bit and removes the need for two wrapper functions.
Fix a caller that tested for == 1 instead of != 0.

gcc/c/ChangeLog:
	* c-decl.cc (diagnose_mismatched_decls): Adapt.
	* c-tree.h (comptypes, comptypes_check_enum_int): Adapt.
	(comptypes_check_different_types): Remove.
	* c-typeck.cc (compatible_types_for_indirection_note_p): Fix.
	(comptypes_internal): Remove outdated info from comment.
	(comptypes, comptypes_check_enum_int): Adapt.
	(comptypes_check_different_types,comp_parm_types): Remove.
	(comp_target_types,build_function_call_vec,digest_init): Adapt.