Auto merge of #115418 - Zoxc:freeze-source, r=oli-obk

Use `Freeze` for `SourceFile`

This uses the `Freeze` type in `SourceFile` to let accessing `external_src` and `lines` be lock-free.

Behavior of `add_external_src` is changed to set `ExternalSourceKind::AbsentErr` on a hash mismatch which matches the documentation. `ExternalSourceKind::Unneeded` was removed as it's unused.

Based on rust-lang/rust#115401.

Author: bors