[mtouch] Speed up marking types referenced in attributes. Fixes #49087. (#1531)

Switch to a new API introduced in Cecil added new API to get all the
attributes in an assembly, without having to traverse (and load) every type
and member.

This makes mtouch significantly faster when computing the list of referenced
assemblies, which is noticable when mtouch later determines nothing else needs
to be done (because nothing changed).

A few tests show that mtouch is now approximately twice as fast:

* A customer test project goes from 2s to 1s.
* A simple test app goes from 1.3s to 0.6.

https://bugzilla.xamarin.com/show_bug.cgi?id=49087

tools/mtouch/Target.cs

@@ -305,34 +305,8 @@ namespace Xamarin.Bundler
 			GetCustomAttributeReferences (assembly, assemblies, exceptions);
 			GetCustomAttributeReferences (main, assemblies, exceptions);
 			if (main.HasTypes) {
-				foreach (var t in main.Types) {
-					GetTypeReferences (t, assemblies, exceptions);
-				}
-			}
-		}
-
-		void GetTypeReferences (TypeDefinition type, HashSet<string> assemblies, List<Exception> exceptions)
-		{
-			GetCustomAttributeReferences (type, assemblies, exceptions);
-			if (type.HasEvents) {
-				foreach (var e in type.Events)
-					GetCustomAttributeReferences (e, assemblies, exceptions);
-			}
-			if (type.HasFields) {
-				foreach (var f in type.Fields)
-					GetCustomAttributeReferences (f, assemblies, exceptions);
-			}
-			if (type.HasMethods) {
-				foreach (var m in type.Methods)
-					GetCustomAttributeReferences (m, assemblies, exceptions);
-			}
-			if (type.HasProperties) {
-				foreach (var p in type.Properties)
-					GetCustomAttributeReferences (p, assemblies, exceptions);
-			}
-			if (type.HasNestedTypes) {
-				foreach (var nt in type.NestedTypes)
-					GetTypeReferences (nt, assemblies, exceptions);
+				foreach (var ca in main.GetCustomAttributes ())
+					GetCustomAttributeReferences (ca, assemblies, exceptions);
 			}
 		}
 
@@ -340,19 +314,23 @@ namespace Xamarin.Bundler
 		{
 			if (!cap.HasCustomAttributes)
 				return;
-			foreach (var ca in cap.CustomAttributes) {
-				if (ca.HasConstructorArguments) {
-					foreach (var arg in ca.ConstructorArguments)
-						GetCustomAttributeArgumentReference (arg, assemblies, exceptions);
-				}
-				if (ca.HasFields) {
-					foreach (var arg in ca.Fields)
-						GetCustomAttributeArgumentReference (arg.Argument, assemblies, exceptions);
-				}
-				if (ca.HasProperties) {
-					foreach (var arg in ca.Properties)
-						GetCustomAttributeArgumentReference (arg.Argument, assemblies, exceptions);
-				}
+			foreach (var ca in cap.CustomAttributes)
+				GetCustomAttributeReferences (ca, assemblies, exceptions);
+		}
+
+		void GetCustomAttributeReferences (CustomAttribute ca, HashSet<string> assemblies, List<Exception> exceptions)
+		{
+			if (ca.HasConstructorArguments) {
+				foreach (var arg in ca.ConstructorArguments)
+					GetCustomAttributeArgumentReference (arg, assemblies, exceptions);
+			}
+			if (ca.HasFields) {
+				foreach (var arg in ca.Fields)
+					GetCustomAttributeArgumentReference (arg.Argument, assemblies, exceptions);
+			}
+			if (ca.HasProperties) {
+				foreach (var arg in ca.Properties)
+					GetCustomAttributeArgumentReference (arg.Argument, assemblies, exceptions);
 			}
 		}