Rollup merge of #41061 - arielb1:parent-lock, r=eddyb

cstore: return an immutable borrow from `visible_parent_map`

This prevents an ICE when `visible_parent_map` is called multiple times, for example when an item referenced in an impl signature is imported from an  `extern crate` statement occurs within an impl.

Fixes #41053.

r? @eddyb

diff --cc src/librustc/middle/cstore.rs
Simple merge

diff --cc src/librustc_metadata/cstore_impl.rs
Simple merge