c++: Stop defining true, false and bool as macros in <stdbool.h>

Since r216679 these macros have only been defined in C++98 mode, rather
than all modes. That is permitted as a GNU extension because that header
doesn't exist in the C++ standard until C++11, so we can make it do
whatever we want for C++98. But as discussed in the PR c++/60304
comments, these macros shouldn't ever be defined for C++.

This patch removes the macro definitions for C++98 too.

The new test already passed for C++98 (and the conversion is ill-formed
in C++11 and later) so this new test is arguably unnecessary.

gcc/ChangeLog:

	PR c++/60304
	* ginclude/stdbool.h (bool, false, true): Never define for C++.

gcc/testsuite/ChangeLog:

	PR c++/60304
	* g++.dg/warn/Wconversion-null-5.C: New test.

gcc/ginclude/stdbool.h

@@ -39,13 +39,6 @@ see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
 /* Supporting _Bool in C++ is a GCC extension.  */
 #define _Bool	bool
 
-#if __cplusplus < 201103L
-/* Defining these macros in C++98 is a GCC extension.  */
-#define bool	bool
-#define false	false
-#define true	true
-#endif
-
 #endif /* __cplusplus */
 
 /* Signal that all the definitions are present.  */

gcc/testsuite/g++.dg/warn/Wconversion-null-5.C

@@ -0,0 +1,6 @@
+// PR c++/60304
+// { dg-do compile { target c++98_only } }
+// { dg-options "-Wconversion-null" }
+
+#include <stdbool.h>
+int * foo() {return false;} // { dg-warning "converting 'false' to pointer" }