[generator] Honor [Internal] on delegates. Fixes #15299. (#19038)

Fixes #15299.

Author: rolfbjarne

src/bgen/Generator.cs

@@ -4799,10 +4799,12 @@ group fullname by ns into g
 				if (AttributeManager.HasAttribute (mi.DeclaringType, "MonoNativeFunctionWrapper"))
 					print ("[MonoNativeFunctionWrapper]\n");
 
-				print ("public delegate {0} {1} ({2});",
+				var accessibility = mi.DeclaringType.IsInternal (this) ? "internal" : "public";
+				print ("{3} delegate {0} {1} ({2});",
 					   RenderType (mi.ReturnType, mi.ReturnTypeCustomAttributes),
 					   shortName,
-					   RenderParameterDecl (mi.GetParameters ()));
+					   RenderParameterDecl (mi.GetParameters ()),
+					   accessibility);
 			}
 
 			if (group.Namespace is not null) {

tests/generator/BGenTests.cs

@@ -1420,6 +1420,12 @@ public void ObsoletedOSPlatform (Profile profile)
 			bgen.AssertExecute ("build");
 		}
 
+		[Test]
+		public void InternalDelegate ()
+		{
+			BuildFile (Profile.iOS, "tests/internal-delegate.cs");
+		}
+
 		BGenTool BuildFile (Profile profile, params string [] filenames)
 		{
 			return BuildFile (profile, true, false, filenames);

tests/generator/tests/internal-delegate.cs

@@ -0,0 +1,15 @@
+using System;
+using Foundation;
+
+namespace NS {
+	[Internal]
+	delegate void Whatever (SomeType value);
+
+	[Internal]
+	[BaseType (typeof (NSObject))]
+	interface SomeType {
+		[Export ("whatever")]
+		Whatever Nope { get; set; }
+	}
+}
+