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

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.