exit: annotate FUNC_ATTR_NORETURN functions #7954 (#7954)

This should fix a particular false positive from clang 5.0.0 scan-build,
which thinks that nlua_init() can continue after preserve_exit().

src/nvim/buffer.c

@@ -4466,7 +4466,7 @@ do_arg_all (
   // leaving an empty tab page when executed locally.
   if (keep_tabs && BUFEMPTY() && curbuf->b_nwindows == 1
       && curbuf->b_ffname == NULL && !curbuf->b_changed) {
-    use_firstwin = TRUE;
+    use_firstwin = true;
   }
 
   for (i = 0; i < count && i < opened_len && !got_int; ++i) {