Mark various language extension flags as deprecated (see #27329)

Adam Gundryrequested to merge
wip/amg/T27329 into master

Changes contained in this patch

The following language extensions are now deprecated, meaning that they will emit a warning with -Wdeprecated-flags:

  • AlternativeLayoutRule
  • AlternativeLayoutRuleTransitional
  • ParallelArrays
  • PolymorphicComponents
  • Rank2Types

In addition, the warning -Walternative-layout-rule-transitional has been marked as deprecated, as it is emitted only under the deprecated extension XAlternativeLayoutRuleTransitional.

MR Checklist

  • This MR solves the problem described in the following issue: #27329
  • A changelog entry was added in changelog.d/ for user-facing changes.
  • This MR does not make any significant changes to base.
  • If this MR has the potential to break user programs, the user-facing label was applied to test against head.hackage.
  • All commits are either individually buildable or squashed.
  • Commit messages describe what they do, referring to tickets using #NNNNN syntax.
  • Source comments describing the change were added. For larger changes [notes][notes] and cross-references from the relevant places were added (as applicable).
  • [Testcases to the testsuite][adding test] were added (as applicable).
  • The users guide was updated (as applicable).

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