Types like `Type::CString` really only assert that at one point the object had its class field equal to `String`. Once a singleton class is created for any strings, the type makes no assertion about any class field anymore, and becomes the same as `Type::TString`. Previously, the `--yjit-verify-ctx` option wasn't allowing objects of these kind that have have singleton classes to pass verification even though the code generators handle it just fine. Found through `ruby/spec`.