Rollup merge of #80734 - abonander:ab/issue-66693, r=oli-obk

check that first arg to `panic!()` in const is `&str`

closes #66693

~~TODO: regression test~~

cc `@RalfJung` for error message wording

diff --cc compiler/rustc_mir/src/transform/check_consts/ops.rs
Simple merge

diff --cc compiler/rustc_mir/src/transform/check_consts/validation.rs
Simple merge