Don't suggest turning on typeclass extensions that don't help

Returning to the example from: https://github.com/haskell/error-messages/issues/42 , when FlexibleInstances is already on we get the not-that-bad error

```
ghci> 1 + [2,3]

:6:1: error:
* No instance for (Num [a0]) arising from a use of `it'
* In a stmt of an interactive GHCi command: print it
```

But when FlexibleInstances is not yet on, we instead get an error suggesting we turn it on!

I think that errors which suggest some typeclass extension should be turned on (at call sites, not instance declarations) should first check if the extension would help -- i.e. if the code would typecheck _with_ the extension. If it still would not typecheck with the extension, then the error message should be about the failure to typecheck, not the extension that requires it.

This seems like a reasonable-to-implement principle which could potentially improve a broad class of error messages.