session: diagnostic migration lint on more fns

Apply the diagnostic migration lint to more functions on `Session`.

Signed-off-by: David Wood <david.wood@huawei.com>

src/lib.rs

@@ -200,6 +200,7 @@ unsafe impl Sync for GccContext {}
 impl WriteBackendMethods for GccCodegenBackend {
     type Module = GccContext;
     type TargetMachine = ();
+    type TargetMachineError = ();
     type ModuleBuffer = ModuleBuffer;
     type ThinData = ();
     type ThinBuffer = ThinBuffer;