Detect and throw error on "broken" GHCs

[martijnbastiaan]

We know some GHC versions interact poorly with Clash on some platforms. Rather than making everyone hunt down the right tickets on their own, this patch makes Clash emit a fat warning error. Because these issues don't completely prevent Clash from working, we allow users to bypass the error by passing a flag -fclash-ignore-broken-ghcs.

Still TODO:

• [x] Write a changelog entry (see changelog/README.md)
• [X] Check copyright notices are up to date in edited files

[leonschoorl]

We have some special handling of certain flags with respect to HDL caching over here. It might be nice to add this option there too, so adding a -fclash-ignore-broken-ghcs doesn't invalidate the HDL cache. And assuming that the code path producing the "broken ghc error" is way before HDL generation, even removing -fclash-ignore-broken-ghcs doesn't have to invalidate the cache.

[martijnbastiaan]

@DigitalBrains1 @leonschoorl Done.