[llvm] [LTO] Turn ImportListsTy into a proper class (NFC) (PR #106427)

[Nikita Popov via llvm-commits]

nikic wrote:

We've encountered an issue when integrating this change into Rust. The problem is that this commit removed the possibility to fetch the import list without potentially modifying the DenseMap. Rust can read the ImportLists concurrently from multiple threads, and this may result in crashes, as reported at https://github.com/rust-lang/rust/pull/129749#issuecomment-2354417960. At least that's my current theory.

Rust was previously using `lookup()` instead of `operator[]`. Would it be possible to restore some form of read-only access to this data structure, whether via lookup or some other method?

https://github.com/llvm/llvm-project/pull/106427