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_const_eval/src/transform/check_consts/qualifs.rs

@@ -148,7 +148,7 @@ impl Qualif for NeedsNonConstDrop {
 
     fn in_any_value_of_ty<'tcx>(cx: &ConstCx<'_, 'tcx>, ty: Ty<'tcx>) -> bool {
         // Avoid selecting for simple cases, such as builtin types.
-        if ty::util::trivial_const_drop(ty) {
+        if ty::util::is_trivially_const_drop(ty) {
             return false;
         }