Remove -Zsaturating-float-casts flag

[hanna-kruppe]

I added this flag back in 2017 (#45205) to enable benchmarking of the saturating semantics when it was newly implemented and still experimental. But saturation has been the official semantics for float<->int as casts for years now. A flag for turning it off no longer serves any purpose, it's just -Zplease-miscompile-casts now.

[rustbot]

miri is developed in its own repository. If the Miri part of this change can be broken out, consider making this change to rust-lang/miri instead. However, if Miri needs adjusting for rustc changes, just ignore this message.

cc @rust-lang/miri

rustc_codegen_cranelift is developed in its own repository. If possible, consider making this change to rust-lang/rustc_codegen_cranelift instead.

cc @bjorn3

[rustbot]

r? @chenyukang

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[hanna-kruppe]

Moved the miri part to rust-lang/miri#5297

The changes to codegen backends can only be separated out by first removing the uses of the flag there, waiting for the next sync, and then removing the rustc flag. That seems unnecessary but will do if the codegen backend maintainers would prefer.

[chenyukang]

@rustbot reroll

[RalfJung]

FWIW it'd have been fine to do the Miri part here. For tiny changes that logically belong to a rustc PR we're not strict. But a separate PR also works. :)

[RalfJung]

Cc @rust-lang/compiler in case someone is attached to this flag.
r? me

r=me if nobody speaks up within the next few days. Thanks for the cleanup!

[bjorn3]

Changing cg_clif in this PR is fine.

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