Foreign types are trivially drop

- Also rename a trivial_const_drop to match style of other functions in
  the util module.
- Also add a test for `const Drop` that doesn't depend on a `~const`
  bound.
- Also comment a bit why we remove the const bound during dropck impl
  check.

compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs

@@ -932,7 +932,6 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             | ty::Bound(..)
             | ty::Param(_)
             | ty::Placeholder(_)
-            | ty::Foreign(_)
             | ty::Projection(_) => {
                 // We don't know if these are `~const Drop`, at least
                 // not structurally... so don't push a candidate.
@@ -951,6 +950,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             | ty::FnDef(..)
             | ty::FnPtr(_)
             | ty::Never
+            | ty::Foreign(_)
             | ty::Array(..)
             | ty::Slice(_)
             | ty::Closure(..)

compiler/rustc_trait_selection/src/traits/select/confirmation.rs

@@ -1133,7 +1133,8 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 | ty::Ref(..)
                 | ty::FnDef(..)
                 | ty::FnPtr(_)
-                | ty::Never => {}
+                | ty::Never
+                | ty::Foreign(_) => {}
 
                 // These types are built-in, so we can fast-track by registering
                 // nested predicates for their constituient type(s)