base: Revert GHC.RTS.Flags changes of !9676

Here we address #25549 (closed), reintroducing Show and Enum instances for IoSubSystem.

added core libraries label

changed milestone to %9.12.1

added 22 commits

• 4dec7d02...f7cbffe2 - 20 commits from branch master
• 49225652 - base: Reintroduce {Show,Enum} IoSubSystem
• ddf88d87 - base: Revert GHC.RTS.Flags changes of !9676 (closed)

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It turns out that this isn't necessary; the majority of the changes were approved in CLC#263. I have dropped all but the relevant commit reintroducing the missing instances.

added 1 commit

• 06f408ee - base: Reintroduce {Show,Enum} IoSubSystem

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changed the description

added full-ci label

added 1 commit

• ca14415a - base: Reintroduce {Show,Enum} IoSubSystem

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assigned to @marge-bot

I will attempt to batch this MR (!13691 (closed))...

Merged in dfd1db48

closed

mentioned in issue #25549 (closed)

added backport needed:9.12 label

@wz1000 this needs to be backported to GHC 9.12.1 please.

mentioned in merge request !13733 (merged)

Backported to %9.12.1 in !13733 (merged)

removed backport needed:9.12 label