[ObjCRuntime] Improve MarkDirty speed by caching the IsCustomType value in our flags byte. (#4128)

* [ObjCRuntime] Improve MarkDirty speed by caching the IsCustomType value in our flags byte.

This makes calling MarkDirty for custom types ~60x times faster.

Reference: https://xamarinhq.slack.com/archives/C03CCJHCF/p1527194568000197

* We only have one bit left to use for IsCustomType.

* Fix build and make enums between native and managed match.

runtime/xamarin/runtime.h

@@ -139,6 +139,8 @@ enum NSObjectFlags {
 	NSObjectFlagsRegisteredToggleRef = 8,
 	NSObjectFlagsInFinalizerQueue = 16,
 	NSObjectFlagsHasManagedRef = 32,
+	// 64, // Used by SoM
+	NSObjectFlagsIsCustomType = 128,
 };
 
 struct AssemblyLocation {

src/Foundation/NSObject2.cs

@@ -79,6 +79,8 @@ namespace Foundation {
 			RegisteredToggleRef = 8,
 			InFinalizerQueue = 16,
 			HasManagedRef = 32,
+			// 64, // Used by SoM
+			IsCustomType = 128,
 		}
 
 		bool disposed { 
@@ -103,6 +105,18 @@ namespace Foundation {
 			get { return ((flags & Flags.InFinalizerQueue) == Flags.InFinalizerQueue); }
 		}
 
+		bool IsCustomType {
+			get {
+				var value = (flags & Flags.IsCustomType) == Flags.IsCustomType;
+				if (!value) {
+					value = Class.IsCustomType (GetType ());
+					if (value)
+						flags |= Flags.IsCustomType;
+				}
+				return value;
+			}
+		}
+
 		[Export ("init")]
 		public NSObject () {
 			bool alloced = AllocIfNeeded ();
@@ -169,7 +183,7 @@ namespace Foundation {
 			if (IsRegisteredToggleRef)
 				return;
 
-			if (!allowCustomTypes && Class.IsCustomType (GetType ()))
+			if (!allowCustomTypes && IsCustomType)
 				return;
 			
 			IsRegisteredToggleRef = true;