Also remove the empty StoredDeclsList entry from the lookup table

[v.g.vassilev]

Summary

In case where we have removed all declarations for a given declaration name entry we should remove the whole StoredDeclsMap entry. This patch improves consistency in the lookup tables and helps cling/clang-repl error recovery.

[v.g.vassilev]

ping.

[martong]

LGTM! And please accept my apologies @v.g.vassilev for taking it months to review this small change.

clang/lib/AST/DeclBase.cpp

if (!ND->getDeclName())
       return;
 
     auto *DC = D->getDeclContext();
     do {
       StoredDeclsMap *Map = DC->getPrimaryContext()->LookupPtr;
       if (Map) {
         StoredDeclsMap::iterator Pos = Map->find(ND->getDeclName());
         assert(Pos != Map->end() && "no lookup entry for decl");
-        Pos->second.remove(ND);
+        StoredDeclsList &List = Pos->second;
+        List.remove(ND);
+        // Clean up the entry if there are no more decls.
+        if (List.isNull())
+          Map->erase(Pos);
       }
     } while (DC->isTransparentContext() && (DC = DC->getParent()));
   }
 }
 
 void DeclContext::addHiddenDecl(Decl *D) {
   assert(D->getLexicalDeclContext() == this &&
          "Decl inserted into wrong lexical context");

clang/unittests/AST/ASTImporterTest.cpp

TEST_P(DeclContextTest,
   L.setHasExternalDecls();
   ASSERT_FALSE(L.getAsList());
   auto Results = L.getLookupResult();
   ASSERT_EQ(1u, std::distance(Results.begin(), Results.end()));
 
   // This asserts in the old implementation.
   DC->removeDecl(A0);
   EXPECT_FALSE(DC->containsDecl(A0));
+
+  // Make sure we do not leave a StoredDeclsList with no entries.
+  DC->removeDecl(A1);
+  ASSERT_EQ(Map->find(A1->getDeclName()), Map->end());
 }
 
 struct ImportFunctionTemplateSpecializations
     : ASTImporterOptionSpecificTestBase {};
 
 TEST_P(ImportFunctionTemplateSpecializations,
        TUshouldNotContainFunctionTemplateImplicitInstantiation) {