Swap DtorckConstraint to DropckConstraint

This change was made as per suspicion that this struct was never renamed after consistent use of DropCk.

This also clarifies the meaning behind the name of this structure.

compiler/rustc_middle/src/query/mod.rs

@@ -549,7 +549,7 @@ rustc_queries! {
 
     query adt_dtorck_constraint(
         key: DefId
-    ) -> Result<&'tcx DtorckConstraint<'tcx>, NoSolution> {
+    ) -> Result<&'tcx DropckConstraint<'tcx>, NoSolution> {
         desc { |tcx| "computing drop-check constraints for `{}`", tcx.def_path_str(key) }
     }