Move 'export name' out of the import section. (#3950)

This implements recent updates to #3938

toolchain/check/handle_export.cpp

@@ -4,6 +4,7 @@
 
 #include "toolchain/check/context.h"
 #include "toolchain/check/decl_name_stack.h"
+#include "toolchain/check/modifiers.h"
 #include "toolchain/parse/typed_nodes.h"
 #include "toolchain/sem_ir/ids.h"
 #include "toolchain/sem_ir/typed_insts.h"
@@ -12,6 +13,7 @@ namespace Carbon::Check {
 
 auto HandleExportIntroducer(Context& context,
                             Parse::ExportIntroducerId /*node_id*/) -> bool {
+  context.decl_state_stack().Push(DeclState::Export);
   // TODO: Probably need to update DeclNameStack to restrict to only namespaces.
   context.decl_name_stack().PushScopeAndStartName();
   return true;
@@ -22,6 +24,10 @@ auto HandleExportDirective(Context& context, Parse::ExportDirectiveId node_id)
   auto name_context = context.decl_name_stack().FinishName();
   context.decl_name_stack().PopScope();
 
+  LimitModifiersOnDecl(context, KeywordModifierSet::None,
+                       Lex::TokenKind::Export);
+  context.decl_state_stack().Pop(DeclState::Export);
+
   if (name_context.state == DeclNameStack::NameContext::State::Error) {
     // Should already be diagnosed.
     return true;