[Foundation] Add not bound NSDateComponentUndefined. Fixes 60740 (#3471)

Added the value and provided tests that show the API usage. Value was
inferred from the headers:

NSDateComponentUndefined = NSIntegerMax

src/Foundation/NSDateComponents.cs

@@ -0,0 +1,7 @@
+using System;
+
+namespace Foundation {
+	public partial class NSDateComponents {
+		public static readonly nint Undefined = nint.MaxValue;
+	}
+}

src/frameworks.sources

@@ -658,6 +658,7 @@ FOUNDATION_CORE_SOURCES = \
 	Foundation/NSAttributedString.iOS.cs \
 	Foundation/EnumDesktop.cs \
 	Foundation/NSObject.mac.cs \
+	Foundation/NSDateComponents.cs \
 
 FOUNDATION_SOURCES = \
 	Foundation/Additions.cs \

tests/monotouch-test/Foundation/NSDateComponentsTest.cs

@@ -0,0 +1,33 @@
+using System;
+#if XAMCORE_2_0
+using Foundation;
+#else
+using MonoTouch.Foundation;
+using MonoTouch.ObjCRuntime;
+#endif
+using NUnit.Framework;
+
+namespace MonoTouchFixtures.Foundation {
+
+	[TestFixture]
+	[Preserve (AllMembers = true)]
+	public class NSDateComponentsTest {
+	
+		[Test]
+		public void TestUndefinedComponent ()
+		{
+			// as per documentation:
+			// "When a new instance of NSDateComponents is created, 
+			// the date components are set to 
+			// NSDateComponentUndefined." 
+			// we simply test that the values are undefined
+			var components = new NSDateComponents ();
+			Assert.AreEqual (NSDateComponents.Undefined, components.Year, $"Year");
+			Assert.AreEqual (NSDateComponents.Undefined, components.Month, "Month");
+			Assert.AreEqual (NSDateComponents.Undefined, components.Day, "Day");
+			Assert.AreEqual (NSDateComponents.Undefined, components.Hour, "Hour");
+			Assert.AreEqual (NSDateComponents.Undefined, components.Minute, "Minute");
+			Assert.AreEqual (NSDateComponents.Undefined, components.Second, "Second");
+		}
+	}
+}