Rollup merge of #118863 - Enselic:rustc_mir-build-query-stability, r=michaelwoerister

rustc_mir_build: Enforce `rustc::potential_query_instability` lint

Stop allowing `rustc::potential_query_instability` on all of `rustc_mir_build` and instead allow it on a case-by-case basis if it is safe to do so. In this crate there was only one instance of the lint, and it was safe to allow.

Part of https://github.com/rust-lang/rust/issues/84447 which is E-help-wanted.