Remove artificial restriction on lifetime parameters on enums

tests/rust/lifetime_arg.rs

@@ -3,6 +3,12 @@ struct A<'a> {
     data: &'a i32
 }
 
+#[repr(C)]
+enum E<'a> {
+    V,
+    U(&'a u8),
+}
+
 #[no_mangle]
-pub extern "C" fn root<'a>(_a: A<'a>)
+pub extern "C" fn root<'a>(_a: A<'a>, _e: E<'a>)
 { }