Merge branch '927-teach-clang-that-_fail-does-not-return' into 'master'

Resolve "teach clang that _fail() does not return."

Closes #927

See merge request isc-projects/bind9!1621

lib/ns/tests/plugin_test.c

@@ -26,10 +26,13 @@
 #include <string.h>
 
 #include <isc/mem.h>
+#include <isc/platform.h>
 #include <isc/result.h>
 #include <isc/types.h>
 #include <isc/util.h>
 
+ISC_PLATFORM_NORETURN_PRE void _fail(const char * const file, const int line) ISC_PLATFORM_NORETURN_POST;
+
 #include <ns/hooks.h>
 
 #include "nstest.h"