Fix dataflow analysis of lambdas in async methods (#122623)

To the same tune as #122450. This is a bug farm, as expected, but likely
still better than trying to teach the code that is shared with ILLink
and Roslyn analyzer about async variants.

Cc @dotnet/ilc-contrib

Author: MichalStrehovsky

src/coreclr/tools/aot/ILCompiler.Compiler/Compiler/Dataflow/CompilerGeneratedState.cs

@@ -737,6 +737,10 @@ public bool TryGetUserMethodForCompilerGeneratedMember(TypeSystemEntity sourceMe
             if (sourceMember == null)
                 return false;
 
+            Debug.Assert(sourceMember is not MethodDesc sourceMethod || sourceMethod.IsTypicalMethodDefinition);
+            if (sourceMember is AsyncMethodVariant asyncVariant)
+                sourceMember = asyncVariant.Target;
+
             TypeSystemEntity member = sourceMember;
             MethodDesc? userMethodCandidate;
             while (TryGetOwningMethodForCompilerGeneratedMember(member, out userMethodCandidate))

src/tools/illink/test/Mono.Linker.Tests.Cases/DataFlow/RuntimeAsyncMethods.cs

@@ -3,6 +3,7 @@
 
 using System;
 using System.Diagnostics.CodeAnalysis;
+using System.Reflection;
 using System.Runtime.CompilerServices;
 using System.Threading.Tasks;
 using Mono.Linker.Tests.Cases.Expectations.Assertions;
@@ -28,6 +29,7 @@ public static async Task Main()
             await RuntimeAsyncWithCorrectParameter(null);
             await RuntimeAsyncWithLocalAll();
             await RuntimeAsyncWithLambda();
+            await RuntimeAsyncWithAwaitedLocalMethod();
         }
 
         static async Task BasicRuntimeAsyncMethod()
@@ -116,5 +118,16 @@ static async Task RuntimeAsyncWithLambda()
         {
             await Task.Run([ExpectedWarning("IL2026", nameof(TypeWithRucMethod.RucMethod))] () => typeof(TypeWithRucMethod).GetMethods());
         }
+
+        static async Task RuntimeAsyncWithAwaitedLocalMethod()
+        {
+            await GetTheMethods();
+
+            [ExpectedWarning("IL2026", nameof(TypeWithRucMethod.RucMethod))]
+            static async Task<MethodInfo[]> GetTheMethods()
+            {
+                return typeof(TypeWithRucMethod).GetMethods();
+            }
+        }
     }
 }