Fix some perf issues in SCM SourceGeneration (Azure#50326)

* Fix some perf issues in SCM SourceGeneration

* Fix WebJobs Storage listener mock failure from latest Storage release that made AccountName virtual

---------

Co-authored-by: tg-msft <[email protected]>

Author: m-nash

sdk/core/System.ClientModel/gen/IncrementalValuesProviderExtensions.cs

@@ -0,0 +1,24 @@
+// Copyright (c) Microsoft Corporation. All rights reserved.
+// Licensed under the MIT License.
+
+using Microsoft.CodeAnalysis;
+
+namespace System.ClientModel.SourceGeneration
+{
+    internal static class IncrementalValuesProviderExtensions
+    {
+        public static IncrementalValuesProvider<TResult> SelectWhen<TInput, TResult>(
+            this IncrementalValuesProvider<TInput> input,
+            IncrementalValueProvider<bool> condition,
+            Func<TInput, TResult?> selector)
+            where TResult : class
+        {
+            return input
+                .Combine(condition)
+                .Where(static tuple => tuple.Right)
+                .Select((tuple, _) => selector(tuple.Left))
+                .Where(static result => result is not null)
+                .Select(static (result, _) => result!);
+        }
+    }
+}