Auto merge of #1863 - hyd-dev:parallel-compiler, r=RalfJung

Use `Lrc` instead of `Rc` in `MiriCompilerCalls::config()`

The `used_crate_source` query defines its return type as [`Lrc<CrateSource>`](https://github.com/rust-lang/rust/blob/1195bea5a7b73e079fa14b37ac7e375fc77d368a/compiler/rustc_middle/src/query/mod.rs#L1418), which would be `Arc` when `parallel-compiler = true`.

I assume this fixes rust-lang/rust#87623. I didn't test that locally -- I'm not interested in `parallel-compiler = true` enough to be willing to recompile rustc with that configuration to test it, but I'm submitting this in case it might be useful for `parallel-compiler = true` users.