Correctly handle rustdoc PlaceholderImplItem in lints

Author: GuillaumeGomez

src/librustdoc/passes/calculate_doc_coverage.rs

@@ -199,7 +199,7 @@ impl DocVisitor<'_> for CoverageCalculator<'_, '_> {
         }
 
         match i.kind {
-            clean::StrippedItem(..) => {
+            clean::StrippedItem(..) | clean::PlaceholderImplItem => {
                 // don't count items in stripped modules
                 return;
             }

src/librustdoc/passes/check_doc_test_visibility.rs

@@ -80,6 +80,7 @@ pub(crate) fn should_have_doc_example(cx: &DocContext<'_>, item: &clean::Item) -
                 | clean::ImplAssocConstItem(..)
                 | clean::RequiredAssocTypeItem(..)
                 | clean::ImplItem(_)
+                | clean::PlaceholderImplItem
         )
     {
         return false;