Slightly reduce allocations in SymbolCompletionItem.AddSymbolInfo (#76418)

* Slightly reduce allocations in SymbolCompletionItem.AddSymbolInfo

This code creates a string from the SymbolKind enum and was showing up as 0.2% of allocations in the typing scenario in the csharp editing speedometer test.

Instead, mimic what the core runtime does in the Number class by keeping a small cache mapping int => string that is lazily populated.

Author: ToddGrun

src/Features/Core/Portable/Completion/Providers/SmallNumberFormatter.cs

@@ -0,0 +1,24 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+namespace Microsoft.CodeAnalysis.Completion.Providers;
+
+internal static class SmallNumberFormatter
+{
+    private const int SmallNumberCacheLength = 32;
+
+    /// <summary>
+    /// Lazily-populated cache of strings for values in the range [0, <see cref="SmallNumberFormatter"/>).
+    /// Inspired by corresponding field in core runtime's Number class. 
+    /// </summary>
+    private static readonly string[] s_smallNumberCache = new string[SmallNumberCacheLength];
+
+    internal static string ToString(int value)
+    {
+        if (value >= SmallNumberCacheLength)
+            return value.ToString();
+
+        return s_smallNumberCache[value] ??= value.ToString();
+    }
+}

src/Features/Core/Portable/Completion/Providers/SymbolCompletionItem.cs

@@ -85,7 +85,7 @@ private static void AddSymbolInfo(ImmutableArray<ISymbol> symbols, ArrayBuilder<
     {
         var symbol = symbols[0];
         var isGeneric = symbol.GetArity() > 0;
-        properties.Add(KeyValuePairUtil.Create("SymbolKind", ((int)symbol.Kind).ToString()));
+        properties.Add(KeyValuePairUtil.Create("SymbolKind", SmallNumberFormatter.ToString((int)symbol.Kind)));
         properties.Add(KeyValuePairUtil.Create("SymbolName", symbol.Name));
 
         if (isGeneric)