Turn the "no saved object file in work product" ICE into a translatable fatal error

compiler/rustc_codegen_ssa/messages.ftl

@@ -190,6 +190,8 @@ codegen_ssa_no_module_named =
 
 codegen_ssa_no_natvis_directory = error enumerating natvis directory: {$error}
 
+codegen_ssa_no_saved_object_file = cached cgu {$cgu_name} should have an object file, but doesn't
+
 codegen_ssa_processing_dymutil_failed = processing debug info with `dsymutil` failed: {$status}
     .note = {$output}
 

compiler/rustc_codegen_ssa/src/back/write.rs

@@ -913,7 +913,9 @@ fn execute_copy_from_cache_work_item<B: ExtraBackendMethods>(
 
     let object = load_from_incr_comp_dir(
         cgcx.output_filenames.temp_path(OutputType::Object, Some(&module.name)),
-        module.source.saved_files.get("o").expect("no saved object file in work product"),
+        module.source.saved_files.get("o").unwrap_or_else(|| {
+            cgcx.create_dcx().emit_fatal(errors::NoSavedObjectFile { cgu_name: &module.name })
+        }),
     );
     let dwarf_object =
         module.source.saved_files.get("dwo").as_ref().and_then(|saved_dwarf_object_file| {

compiler/rustc_codegen_ssa/src/errors.rs

@@ -121,6 +121,12 @@ pub struct NoNatvisDirectory {
     pub error: Error,
 }
 
+#[derive(Diagnostic)]
+#[diag(codegen_ssa_no_saved_object_file)]
+pub struct NoSavedObjectFile<'a> {
+    pub cgu_name: &'a str,
+}
+
 #[derive(Diagnostic)]
 #[diag(codegen_ssa_copy_path_buf)]
 pub struct CopyPathBuf {