Auto merge of #126757 - compiler-errors:safe, r=spastorino

Properly gate `safe` keyword in pre-expansion

This PR gates `safe` keyword in pre-expansion contexts. Should mitigate the fallout of rust-lang/rust#126755, which is that `safe` is now usable on beta lol.

r? `@spastorino` or `@oli-obk`

cc #124482 tracking #123743

Author: bors