[xamlc] return early in `NodeILExtensions` for `ValidateOnly` (#24493)

Context: 7efca8efb3
Context: https://github.com/xamarin/Xamarin.Forms/pull/7407

For Debug builds, .NET MAUI runs XamlC in a "ValidateOnly" mode, which
avoids writing `.dll` files at the end. This way, we get faster
builds, but still emit the same errors and warnings that would be
emitted in `Release` mode.

Right now, the `ValidateOnly` flag is only checked at the end of the
process to skip writing files. I looked for some places taking time,
such as:

    36.30ms (1.1%) microsoft.maui.controls.build.tasks!Microsoft.Maui.Controls.Build.Tasks.NodeILExtensions+<PushServiceProvider>d__12.MoveNext()
    20.20ms (0.6%) microsoft.maui.controls.build.tasks!Microsoft.Maui.Controls.Build.Tasks.NodeILExtensions+<PushParentObjectsArray>d__9.MoveNext()
     1.21ms (0.0%) microsoft.maui.controls.build.tasks!Microsoft.Maui.Controls.Build.Tasks.NodeILExtensions.PushXmlLineInfo(class Microsoft.Maui.Controls.Xaml.INode,class Microsoft.Maui.Controls.Build.Tasks.ILContext)

And added checks to just return early in these methods.

With this change in-place, an incremental build of a `dotnet new maui`
project template:

    Before:
    1704 ms  XamlCTask                                  4 calls
    After:
    1402 ms  XamlCTask                                  4 calls

This probably saves a little less than 100ms per call, but in this
example it is running once per platform. This improvement probably
will help even more for larger projects with many XAML files.

src/Controls/src/Build.Tasks/ILContext.cs

@@ -44,5 +44,7 @@ namespace Microsoft.Maui.Controls.Build.Tasks
 		public string XamlFilePath { get; internal set; }
 
 		public TaskLoggingHelper LoggingHelper { get; internal set; }
+
+		public bool ValidateOnly { get; set; }
 	}
 }

src/Controls/src/Build.Tasks/NodeILExtensions.cs

@@ -422,6 +422,10 @@ namespace Microsoft.Maui.Controls.Build.Tasks
 
 		public static IEnumerable<Instruction> PushXmlLineInfo(this INode node, ILContext context)
 		{
+			if (context.ValidateOnly)
+			{
+				yield break;
+			}
 			var module = context.Body.Method.Module;
 
 			var xmlLineInfo = node as IXmlLineInfo;
@@ -447,6 +451,10 @@ namespace Microsoft.Maui.Controls.Build.Tasks
 
 		public static IEnumerable<Instruction> PushParentObjectsArray(this INode node, ILContext context)
 		{
+			if (context.ValidateOnly)
+			{
+				yield break;
+			}
 			var module = context.Body.Method.Module;
 
 			var nodes = new List<IElementNode>();
@@ -585,6 +593,10 @@ namespace Microsoft.Maui.Controls.Build.Tasks
 
 		public static IEnumerable<Instruction> PushServiceProvider(this INode node, ILContext context, FieldReference bpRef = null, PropertyReference propertyRef = null, TypeReference declaringTypeReference = null)
 		{
+			if (context.ValidateOnly)
+			{
+				yield break;
+			}
 			var module = context.Body.Method.Module;
 
 #if NOSERVICEPROVIDER

src/Controls/src/Build.Tasks/SetPropertiesVisitor.cs

@@ -1666,6 +1666,7 @@ namespace Microsoft.Maui.Controls.Build.Tasks
 				Root = root,
 				XamlFilePath = parentContext.XamlFilePath,
 				LoggingHelper = parentContext.LoggingHelper,
+				ValidateOnly = parentContext.ValidateOnly,
 			};
 
 			//Instanciate nested class

src/Controls/src/Build.Tasks/XamlCTask.cs

@@ -354,6 +354,7 @@ namespace Microsoft.Maui.Controls.Build.Tasks
 				{
 					XamlFilePath = xamlFilePath,
 					LoggingHelper = loggingHelper,
+					ValidateOnly = ValidateOnly,
 				};