Fix for #21456 (Regressions in attribute allocations for non-generic …

…attributes) (#21462)

* Fix for #21456 - restrict increased generic attribute allocations to only generic attributes

This is a trivial quick-fix for #21456 where regressions between 2.1 and 3.0 were discovered on most attibute pathways due to the allocation overhead in the generic-supporting pathways. The workaround is to simply not take that slow/expensive path for non-generics.

While I'd like to optimize `RuntimeModule.ResolveMethod` further, there's a public surface area in play there that makes the changes non-trivial. There, we'll have to choose overhead on the public path (which may still be a net win), or duplication in code for another path.

* Update comments

src/System.Private.CoreLib/src/System/Reflection/CustomAttribute.cs

@@ -1616,7 +1616,15 @@ private static unsafe bool FilterCustomAttributeRecord(
             if (ctorHasParameters)
             {
                 // Resolve method ctor token found in decorated decoratedModule scope
-                ctor = decoratedModule.ResolveMethod(caRecord.tkCtor, attributeType.GenericTypeArguments, null).MethodHandle.GetMethodInfo();
+                // See https://github.com/dotnet/coreclr/issues/21456 for why we fast-path non-generics here (fewer allocations)
+                if (attributeType.IsGenericType)
+                {
+                    ctor = decoratedModule.ResolveMethod(caRecord.tkCtor, attributeType.GenericTypeArguments, null).MethodHandle.GetMethodInfo();
+                }
+                else
+                {
+                    ctor = ModuleHandle.ResolveMethodHandleInternal(decoratedModule.GetNativeHandle(), caRecord.tkCtor);
+                }
             }
             else
             {