Auto merge of #151015 - JonathanBrouwer:fix-cratelevel-perf, r=jdonszelmann

Fix perf of `check_crate_level` refactor

Fixes the perf of rust-lang/rust#150930
The problem is that `allowed_targets` allocates a `Vec`

(also moves a comment around to a move useful place)

r? @jdonszelmann

Author: bors