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

[saethlin]

#124895 is adding a number of FIXME comments on #[allow(static_mut_refs)] in the compiler. This issue tracks fixing them.

See #124895 (comment)

[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]

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

[workingjubilee]

GH API design sure includes some... choices.