C++: suggest missing headers for implicit use of "std" (PR c++/85021)

C++: suggest missing headers for implicit use of "std" (PR c++/85021)

We provide fix-it hints for the most common "std" names when an explicit
"std::" prefix is present, however we don't yet provide fix-it hints for
this implicit case:

  using namespace std;
  void f() {  cout << "test"; }

for which we emit:

  t.cc: In function 'void f()':
  t.cc:2:13: error: 'cout' was not declared in this scope
  void f() {  cout << "test"; }
              ^~~~

This patch detects if a "using namespace std;" directive is present
in the current namespace, and if so, offers a suggestion for
unrecognized names that are in our list of common "std" names:

  t.cc: In function 'void f()':
  t.cc:2:13: error: 'cout' was not declared in this scope
   void f() {  cout << "test"; }
               ^~~~
  t.cc:2:13: note: 'std::cout' is defined in header '<iostream>'; did you forget to '#include <iostream>'?
  +#include <iostream>
   using namespace std;
   void f() {  cout << "test"; }
               ^~~~

gcc/cp/ChangeLog:
	PR c++/85021
	* name-lookup.c (using_directives_contain_std_p): New function.
	(has_using_namespace_std_directive_p): New function.
	(suggest_alternatives_for): Simplify if/else logic using early
	returns.  If no candidates were found, and there's a
	"using namespace std;" directive, call
	maybe_suggest_missing_std_header.
	(maybe_suggest_missing_header): Split later part of the function
	into..
	(maybe_suggest_missing_std_header): New.

gcc/testsuite/ChangeLog:
	PR c++/85021
	* g++.dg/lookup/missing-std-include-7.C: New test.

From-SVN: r259179