Rollup merge of #120135 - oli-obk:smir_private, r=celinval

SMIR: Make the remaining "private" fields actually private

Turns out we have already created a trait that allows us to make the fields private: https://doc.rust-lang.org/nightly/nightly-rustc/stable_mir/ty/trait.IndexedVal.html

fixes rust-lang/project-stable-mir#56

r? `@celinval`

Author: matthiaskrgr