[Macros] Ban extension macros on local nested types

Make sure we check whether we are in any local parent context.

Author: hamishknight

lib/Sema/TypeCheckMacros.cpp

@@ -2093,7 +2093,7 @@ std::optional<unsigned> swift::expandExtensions(CustomAttr *attr,
                                                 MacroDecl *macro,
                                                 MacroRole role,
                                                 NominalTypeDecl *nominal) {
-  if (nominal->getDeclContext()->isLocalContext()) {
+  if (nominal->getLocalContext()) {
     nominal->diagnose(diag::local_extension_macro);
     return std::nullopt;
   }

test/Macros/macro_expand_extensions.swift

@@ -115,6 +115,12 @@ func testLocal() {
   @DelegatedConformance
   struct Local<Element> {}
   // expected-error@-1{{local type cannot have attached extension macro}}
+
+  struct S {
+    @DelegatedConformance
+    struct Local<Element> {}
+    // expected-error@-1{{local type cannot have attached extension macro}}
+  }
 }
 
 @DelegatedConformance