Auto merge of #150353 - llogiq:rustc-hash-refactor, r=eholk

refactor rustc-hash integration

I found that rustc-hash is used in multiple compiler crates. Also some types use `FxBuildHasher` whereas others use `BuildHasherDefault<FxHasher>` (both do the same thing).

In order to simplify future hashing experiments, I changed every location to use `rustc_data_structures::fx::*` types instead, and also removed the `BuildHasherDefault` variant. This will simplify future experiments with hashing (for example trying out a hasher that doesn't implement `Default` for whatever reason).

Author: bors