Add unstable type_ascribe macro

This macro serves as a placeholder for future type ascription syntax to
make sure that the semantic implementation keeps working.

compiler/rustc_span/src/symbol.rs

@@ -1485,6 +1485,7 @@ symbols! {
         ty,
         type_alias_enum_variants,
         type_alias_impl_trait,
+        type_ascribe,
         type_ascription,
         type_changing_struct_update,
         type_id,