Only cache local adt_def results on disk

An `AdtDef` contains `Span`s, and we don't appear
to actually encode the `SourceFile` for foreign
spans in the incremental cache in some cases.

compiler/rustc_middle/src/query/mod.rs

@@ -522,7 +522,7 @@ rustc_queries! {
     }
     query adt_def(key: DefId) -> &'tcx ty::AdtDef {
         desc { |tcx| "computing ADT definition for `{}`", tcx.def_path_str(key) }
-        cache_on_disk_if { true }
+        cache_on_disk_if { key.is_local() }
         separate_provide_extern
     }
     query adt_destructor(key: DefId) -> Option<ty::Destructor> {