Rollup merge of #148647 - JonathanBrouwer:unsafe_attr_refactor, r=jdonszelmann

Check unsafety for non-macro attributes in `validate_attr`

r? `````@jdonszelmann`````

Also adds a test for a previously untested case, unnecessary unsafe on a proc macro attribute

In preparation for rust-lang/rust#148453

Author: Zalathar