Turn long-deprecated -C options into errors

[RalfJung]

Proposal

We have some -C flags that have had no effect other than issuing a warning for a while:

• -Car has been documented to do nothing for more than 8 years (rust-lang/rust@691ab6c) and causes a warning for more than a year (mark deprecated option as deprecated in rustc_session to remove copypasta and small refactor rust#135126).
• -Cno-stack-check has been made a NOP and deprecated with a warning more than 9 years ago (rust-lang/rust@c670293)
• -Cinline-threshold has been made a NOP and deprecated with a warning almost 2 years ago (Deprecate no-op codegen option -Cinline-threshold=... rust#124712).

I think there's no reason to keep these around -- and it risks people using them without reading the docs properly and then getting unexpected results. So I propose we turn these warning into errors: rust-lang/rust#154498.

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[rustbot]

Important

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cc @rust-lang/compiler

[oli-obk]

@rfcbot fcp merge compiler

[rust-rfcbot]

Team member @oli-obk has proposed to merge this. The next step is review by the rest of the tagged team members:

• @Mark-Simulacrum
• @SparrowLii
• @TaKO8Ki
• @cjgillot
• @davidtwco
• @estebank
• @jieyouxu
• @matthewjasper
• @nagisa
• @oli-obk
• @petrochenkov
• @spastorino
• @wesleywiser

No concerns currently listed.

Once a majority of reviewers approve (and at most 2 approvals are outstanding), this will enter its final comment period. If you spot a major issue that hasn't been raised at any point in this process, please speak up!

See this document for info about what commands tagged team members can give me.

[RalfJung]

@SparrowLii @TaKO8Ki @davidtwco @estebank @spastorino @wesleywiser friendly FCP reminder :)