Import SourceFiles from crate before decoding foreign Span

Fixes #92163
Fixes #92014

When writing to the incremental cache, we encode all `Span`s
we encounter, regardless of whether or not their `SourceFile`
comes from the local crate, or from a foreign crate.

When we decode a `Span`, we use the `StableSourceFileId` we encoded
to locate the matching `SourceFile` in the current session. If this
id corresponds to a `SourceFile` from another crate, then we need to
have already imported that `SourceFile` into our current session.

This usually happens automatically during resolution / macro expansion,
when we try to resolve definitions from other crates. In certain cases,
however, we may try to load a `Span` from a transitive dependency
without having ever imported the `SourceFile`s from that crate, leading
to an ICE.

This PR fixes the issue by calling `imported_source_files()`
when we encounter a `SourceFile` with a foreign `CrateNum`.
This ensures that all `SourceFile`s from that crate are imported
into the current session.

compiler/rustc_session/src/cstore.rs

@@ -201,6 +201,12 @@ pub trait CrateStore: std::fmt::Debug {
         index_guess: u32,
         hash: ExpnHash,
     ) -> ExpnId;
+
+    /// Imports all `SourceFile`s from the given crate into the current session.
+    /// This normally happens automatically when we decode a `Span` from
+    /// that crate's metadata - however, the incr comp cache needs
+    /// to trigger this manually when decoding a foreign `Span`
+    fn import_source_files(&self, sess: &Session, cnum: CrateNum);
 }
 
 pub type CrateStoreDyn = dyn CrateStore + sync::Sync;