Replace uses of #[allow(static_mut_refs)] with SyncUnsafeCell

[saethlin]

https://github.com/rust-lang/rust/pull/124895 is adding a number of FIXME comments on #[allow(static_mut_refs)] in the compiler. This issue tracks fixing them.

See https://github.com/rust-lang/rust/pull/124895#issuecomment-2106084818

[workingjubilee]

Hm, I wonder if it works like...

cc @obeis

[RalfJung]

As before, I'm not sure SyncUnsafeCell is the right solution. In many cases it should probably just be raw pointers.

[RalfJung]

@rustbot assign @obeis

[obeis]

@rustbot claim

[RalfJung]

:thinking: I thought claim is just an alias for assign <me>... very strange. Oh well.

[workingjubilee]

GH API design sure includes some... choices.