[ASTGen] Fix nullability

For 'arbitrary' macro names

Author: rintaro

include/swift/AST/ASTBridging.h

@@ -176,7 +176,7 @@ BridgedDeclBaseName
 BridgedDeclBaseName_createIdentifier(BridgedIdentifier identifier);
 
 class BridgedDeclNameRef {
-  void *_Nonnull opaque;
+  void *_Nullable opaque;
 
 public:
   BRIDGED_INLINE BridgedDeclNameRef();

test/ASTGen/macros.swift

@@ -110,3 +110,8 @@ func remoteCall<Result: ConjureRemoteValue>(function: String, arguments: [String
 
 @Remote
 func f(a: Int, b: String) async throws -> String
+
+@freestanding(declaration, names: arbitrary) macro bitwidthNumberedStructs(_ baseName: String) = #externalMacro(module: "MacroDefinition", type: "DefineBitwidthNumberedStructsMacro")
+struct TestArbitrary {
+  #bitwidthNumberedStructs("MyIntOne")
+}