If ‘verify’ fails in a deeply-nested macro, GCC does not output a
useful line number containing the top-level caller of the macro.
So, bring back the older way of issuing a diagnostic containing
the top-level call’s arg, so that it is easier to diagnose
‘verify’ failures with recent GCC.
* lib/verify.h (_GL_VERIFY_TRUE, _GL_VERIFY_TYPE):
Bring back DIAGNOSTIC arg. All callers changed.
(verify): Just use _GL_VERIFY.