Clean up dropck code a bit

- Remove `Result` that couldn't be Err on valid compilation.
- Always compute errors on failure.

compiler/rustc_middle/src/query/mod.rs

@@ -802,7 +802,7 @@ rustc_queries! {
 
     query adt_dtorck_constraint(
         key: DefId
-    ) -> Result<&'tcx DropckConstraint<'tcx>, NoSolution> {
+    ) -> &'tcx DropckConstraint<'tcx> {
         desc { |tcx| "computing drop-check constraints for `{}`", tcx.def_path_str(key) }
     }