Add namespace-scoped private keyword support. (#4026)

Adds access to the name lookup table in name scopes. This is so that we
can quickly check access during name lookup without resolving the entity
itself. Does this for names in general, but does not implement handling
for entity-scoped names, only namespace-scoped names (where they're
essentially just not exported).

Excludes `private` names from exports. Although names should be
accessible to `impl` files, that's not implemented here because we'll
probably want to do it by directly copying name lookup tables.

toolchain/check/handle_function.cpp

@@ -228,10 +228,6 @@ static auto BuildFunctionDecl(Context& context,
       context.decl_introducer_state_stack().Pop<Lex::TokenKind::Fn>();
   DiagnoseModifiers(context, introducer, is_definition, parent_scope_inst_id,
                     parent_scope_inst);
-  if (introducer.modifier_set.HasAnyOf(KeywordModifierSet::Access)) {
-    context.TODO(introducer.modifier_node_id(ModifierOrder::Access),
-                 "access modifier");
-  }
   bool is_extern = introducer.modifier_set.HasAnyOf(KeywordModifierSet::Extern);
   if (introducer.modifier_set.HasAnyOf(KeywordModifierSet::Method)) {
     context.TODO(introducer.modifier_node_id(ModifierOrder::Decl),
@@ -268,6 +264,8 @@ static auto BuildFunctionDecl(Context& context,
   // Create a new function if this isn't a valid redeclaration.
   if (!function_decl.function_id.is_valid()) {
     function_decl.function_id = context.functions().Add(function_info);
+  } else {
+    // TODO: Validate that the redeclaration doesn't set an access modifier.
   }
   function_decl.type_id = context.GetFunctionType(function_decl.function_id);
 
@@ -288,7 +286,8 @@ static auto BuildFunctionDecl(Context& context,
       }
     }
 
-    context.decl_name_stack().AddName(name_context, lookup_result_id);
+    context.decl_name_stack().AddName(name_context, lookup_result_id,
+                                      introducer.modifier_set.GetAccessKind());
   }
 
   if (SemIR::IsEntryPoint(context.sem_ir(), function_decl.function_id)) {