Allow preinitializing EventSource (#101573)

We currently don't preinitialize any event sources because they have a finalizer. This is a bit of a problem for places like this: https://github.com/dotnet/runtime/blob/5c01ed22b7468a2bee13b498855dcfcc5ae4da50/src/libraries/System.Private.CoreLib/src/System/Runtime/CompilerServices/AsyncTaskMethodBuilderT.cs#L229-L232. We know `IsSupported` is false so we eliminate the branch, but we still need a cctor check because we access a static. The cctor check in a generic method that is instantiated many many many times becomes a death by a thousand papercuts.

Saves 0.7% in size on the stage2 app with EventSource disabled.

Author: MichalStrehovsky

src/coreclr/tools/aot/ILCompiler.Compiler/Compiler/TypePreinit.cs

@@ -533,8 +533,14 @@ private Status TryScanMethod(MethodIL methodIL, Value[] parameters, Stack<Method
 
                             if (owningType.HasFinalizer)
                             {
-                                // Finalizer might have observable side effects
-                                return Status.Fail(methodIL.OwningMethod, opcode, "Finalizable class");
+                                // We have a finalizer. There's still a small chance it has been nopped out
+                                // with a feature switch. Check for that.
+                                byte[] finalizerMethodILBytes = _ilProvider.GetMethodIL(owningType.GetFinalizer()).GetILBytes();
+                                if (finalizerMethodILBytes.Length != 1 || finalizerMethodILBytes[0] != (byte)ILOpcode.ret)
+                                {
+                                    // Finalizer might have observable side effects
+                                    return Status.Fail(methodIL.OwningMethod, opcode, "Finalizable class");
+                                }
                             }
 
                             if (_flowAnnotations.RequiresDataflowAnalysisDueToSignature(ctor))

src/libraries/System.Private.CoreLib/src/ILLink/ILLink.Substitutions.Shared.xml

@@ -2,6 +2,7 @@
   <assembly fullname="System.Private.CoreLib">
     <type fullname="System.Diagnostics.Tracing.EventSource" feature="System.Diagnostics.Tracing.EventSource.IsSupported" featurevalue="false">
       <method signature="System.Boolean get_IsSupported()" body="stub" value="false" />
+      <method signature="System.Void Finalize()" body="stub" />
       <method signature="System.Boolean IsEnabled()" body="stub" value="false" />
       <method signature="System.Boolean IsEnabled(System.Diagnostics.Tracing.EventLevel,System.Diagnostics.Tracing.EventKeywords)" body="stub" value="false" />
       <method signature="System.Boolean IsEnabled(System.Diagnostics.Tracing.EventLevel,System.Diagnostics.Tracing.EventKeywords,System.Diagnostics.Tracing.EventChannel)" body="stub" value="false" />

src/libraries/System.Private.CoreLib/src/System/Diagnostics/Tracing/EventSource.cs

@@ -1461,6 +1461,8 @@ public void Dispose()
         /// <param name="disposing">True if called from Dispose(), false if called from the finalizer.</param>
         protected virtual void Dispose(bool disposing)
         {
+            // NOTE: If !IsSupported, we use ILLink.Substitutions to nop out the finalizer.
+            //       Do not add any code before this line (or you'd need to delete the substitution).
             if (!IsSupported)
             {
                 return;
@@ -1504,6 +1506,7 @@ protected virtual void Dispose(bool disposing)
         /// </summary>
         ~EventSource()
         {
+            // NOTE: we nop out this method body if !IsSupported using ILLink.Substitutions.
             this.Dispose(false);
         }
 #endregion