[Xamarin.Android.Build.Tasks] Preserve Debugger Async Supports (#875)

Fixes: https://bugzilla.xamarin.com/show_bug.cgi?id=59516
Context: https://bugzilla.xamarin.com/show_bug.cgi?id=59015

The linker requires that the target process contain members such as
`System.Threading.Tasks.Task.NotifyDebuggerOfWaitCompletion()`.
If these members are missing, the runtime may abort:

	* Assertion at …xamarin-android/external/mono/mono/mini/debugger-agent.c:4765, condition `array->len == 1' not met
	[libc] Fatal signal 6 (SIGABRT), code -6 in tid 11492 (…)

Update the linker so that `async`-related Debugger support
infrastructure is preserved, so that the debugger can rely on them.

*Note*: This commit does *not* fix the assert reported in Bug #59015
on Android as that happens when the linker is disabled as well.
Fixing the assert from Bug #59015 will require another fix.

Author: radekdoulik

src/Xamarin.Android.Build.Tasks/Linker/MonoDroid.Tuner/MonoDroidMarkStep.cs

@@ -140,8 +140,14 @@ protected override TypeDefinition MarkType (TypeReference reference)
 			if (type == null)
 				return null;
 
-			if (type.Module.Assembly.Name.Name == "System.Core")
+			switch (type.Module.Assembly.Name.Name) {
+			case "mscorlib":
+				ProcessCorlib (type);
+				break;
+			case "System.Core":
 				ProcessSystemCore (type);
+				break;
+			}
 
 			if (type.HasMethods && type.HasInterfaces && type.Implements (ICustomMarshalerName)) {
 				foreach (MethodDefinition method in type.Methods) {
@@ -155,6 +161,40 @@ protected override TypeDefinition MarkType (TypeReference reference)
 			return type;
 		}
 
+		bool DebugBuild {
+			get { return _context.LinkSymbols; }
+		}
+
+		void ProcessCorlib (TypeDefinition type)
+		{
+			switch (type.Namespace) {
+			case "System.Runtime.CompilerServices":
+				switch (type.Name) {
+				case "AsyncTaskMethodBuilder":
+					if (DebugBuild) {
+						MarkNamedMethod (type, "SetNotificationForWaitCompletion");
+						MarkNamedMethod (type, "get_ObjectIdForDebugger");
+					}
+					break;
+				case "AsyncTaskMethodBuilder`1":
+					if (DebugBuild) {
+						MarkNamedMethod (type, "SetNotificationForWaitCompletion");
+						MarkNamedMethod (type, "get_ObjectIdForDebugger");
+					}
+					break;
+				}
+				break;
+			case "System.Threading.Tasks":
+				switch (type.Name) {
+				case "Task":
+					if (DebugBuild)
+						MarkNamedMethod (type, "NotifyDebuggerOfWaitCompletion");
+					break;
+				}
+				break;
+			}
+		}
+
 		void ProcessSystemCore (TypeDefinition type)
 		{
 			switch (type.Namespace) {