* c-decl.c (duplicate_decls): Break apart into...
	(diagnose_arglist_conflict, validate_proto_after_old_defn)
	(locate_old_defn, diagnose_mismatched_decls, merge_decls):
	... these new functions.  Restructure for comprehensibility.
	Remove various archaic special cases.  Always report the
	location of the previous declaration when a diagnostic is issued.
	(redeclaration_error_message): Fold into diagnose_mismatched_decls.
	(match_builtin_function_types): Delete unnecessary forward declaration.
testsuite:
	* gcc.dg/Wshadow-1.c, gcc.dg/attr-noinline.c, gcc.dg/decl3.c
	* gcc.dg/redecl-1.c, gcc.dg/visibility-7.c, gcc.dg/wtr-static-1.c
	* gcc.dg/noncompile/20020220-1.c, objc.dg/method-1.m:
	Update dg-error regexps.

From-SVN: r75667