C++: fix ordering of missing std #include suggestion (PR c++/81514)

C++: fix ordering of missing std #include suggestion (PR c++/81514)

gcc/cp/ChangeLog:
	PR c++/81514
	* name-lookup.c (maybe_suggest_missing_header): Convert return
	type from void to bool; return true iff a suggestion was offered.
	(suggest_alternative_in_explicit_scope): Move call to
	maybe_suggest_missing_header to before use of best_match, and
	return true if the former offers a suggestion.

gcc/testsuite/ChangeLog:
	PR c++/81514
	* g++.dg/lookup/empty.h: New file.
	* g++.dg/lookup/missing-std-include-2.C: Replace include of
	stdio.h with empty.h and a declaration of a "std::sprintf" not based
	on a built-in.

From-SVN: r251186