Fixes #18966 (closed). Adds a new warning
-Wambiguous-fields for uses of field selectors or record updates that will be rejected in the future, when the
DuplicateRecordFields extension is simplified per https://github.com/ghc-proposals/ghc-proposals/pull/366.
I'd appreciate feedback on the warning texts. In particular, would it make sense to link to the GHC proposal or some other description of the migration plan from the warning message itself?