re PR c++/39859 (duplicated and unhelpful error for "c:n" (parser))

gcc/cp/
	PR c++/39859
	PR c++/44522
	PR c++/44523
	* parser.c (struct cp_parser): Add colon_corrects_to_scope_p field.
	(cp_parser_new): Initialize it.
	(cp_parser_nested_name_specifier_opt): Auto-correct colons to
	scopes if we are able to.
	(cp_parser_question_colon_clause): Disallow colon correction.
	(cp_parser_label_for_labeled_statement): Likewise.
	(cp_parser_range_for): Likewise.
	(cp_parser_enum_specifier): Likewise.
	(cp_parser_class_head): Likewise.
	(cp_parser_member_declaration): Likewise.

gcc/testsuite/
	PR c++/39859
	PR c++/44522
	PR c++/44523
	* g++.dg/parse/colon-autocorrect-1.C: New testcase.
	* g++.dg/parse/colon-autocorrect-2.C: New testcase.

From-SVN: r167895