Auto merge of #75278 - cuviper:indexmap, r=Mark-Simulacrum

Upgrade indexmap and use it more

First this upgrades `indexmap` to 1.5.1, which is now based on `hashbrown::raw::RawTable`. This means it shares a lot of the same performance characteristics for insert, lookup, etc., while keeping items in insertion order.

Then across various rustc crates, this replaces a lot of `Vec`+`HashMap` pairs with a single `IndexMap` or `IndexSet`.

Closes #60608.
r? @eddyb

diff --cc src/librustc_metadata/rmeta/encoder.rs
Simple merge