Rollup merge of #110632 - saethlin:panic-if-dep-graph-too-big, r=lcnr

Panic instead of truncating if the incremental on-disk cache is too big

It seems _unlikely_ that anyone would hit this truncation, but if this `as` does actually truncate, that seems incredibly bad.

compiler/rustc_query_impl/src/on_disk_cache.rs

@@ -300,7 +300,7 @@ fn serialize(&self, tcx: TyCtxt<'_>, encoder: FileEncoder) -> FileEncodeResult {
                     interpret_alloc_index.reserve(new_n - n);
                     for idx in n..new_n {
                         let id = encoder.interpret_allocs[idx];
-                        let pos = encoder.position() as u32;
+                        let pos: u32 = encoder.position().try_into().unwrap();
                         interpret_alloc_index.push(pos);
                         interpret::specialized_encode_alloc_id(&mut encoder, tcx, id);
                     }