Transition the compiler arenas #61838

We are in the midst of simplifying the compiler's lifetime system. In general, this means that TyCtxt<'a, 'gcx, 'tcx> is going to become just TyCtxt<'tcx>, where 'tcx represents the lifetime of the entire compilation (we no longer have global and local arenas).

Implementation plan

• remove the arenas (link to @Zoxc's PR?)
• warn on unused lifetimes Add deny(unused_lifetimes) to all the crates that have deny(internal). #61735
• rewrite TyCtxt<'a, 'gcx, 'tcx> to TyCtxt<'gcx, 'tcx> rustc: replace TyCtxt<'a, 'gcx, 'tcx> with TyCtxt<'gcx, 'tcx>. #61722
• rewrite TyCtxt<'gcx, 'tcx> to TyCtxt<'tcx> Unify all uses of 'gcx and 'tcx. #61817

If your PR is affected

Once #61817 lands, what you want to do is generally rewrite to TyCtxt<'tcx> or -- less often -- TyCtxt<'_>. More specifically, you can convert older code by pattern matching like so:

• TyCtxt<_, 'tcx, _> becomes TyCtxt<'tcx>
• TyCtxt<'_, '_, 'tcx> becomes TyCtxt<'tcx>

You can also handle any unused lifetime warnings by replacing the unused lifetime with '_.

Note: when adapting your code, you should never need to introduce a new lifetime, you're only removing them. If you find yourself adding a 'gcx to some impl, that's wrong =)