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