Rollup merge of #107740 - oli-obk:lock_tcx, r=petrochenkov

Avoid locking the global context across the `after_expansion` callback

r? `@petrochenkov`

This was noticed in model-checking/kani#2184 (comment)

This didn't have a perf impact, as it's just an additional 2 or 3 RefCell locks being created.