Adjust regression test

Author: JohnTitor

src/test/ui/issues/auxiliary/issue-63226.rs renamed to src/test/ui/consts/auxiliary/issue-63226.rs

@@ -2,6 +2,8 @@
 // compile-flags:--extern issue_63226
 // edition:2018
 // build-pass
+// A regression test for issue #63226.
+// Checks if `const fn` is marked as reachable.
 
 use issue_63226::VTable;