Reduce allocations in NamespaceSymbolKey.Resolve

Related to https://devdiv.visualstudio.com/DevDiv/_workitems/edit/2136177

Author: sharwell

src/Compilers/Core/Portable/PublicAPI.Unshipped.txt

@@ -6,6 +6,7 @@ Microsoft.CodeAnalysis.GeneratorFilterContext
 Microsoft.CodeAnalysis.GeneratorFilterContext.CancellationToken.get -> System.Threading.CancellationToken
 Microsoft.CodeAnalysis.GeneratorFilterContext.Generator.get -> Microsoft.CodeAnalysis.ISourceGenerator!
 Microsoft.CodeAnalysis.GeneratorFilterContext.GeneratorFilterContext() -> void
+Microsoft.CodeAnalysis.INamespaceSymbol.GetMembers<TArg>(string! name, System.Action<Microsoft.CodeAnalysis.INamespaceOrTypeSymbol!, TArg>! callback, TArg argument) -> void
 Microsoft.CodeAnalysis.IPropertySymbol.PartialDefinitionPart.get -> Microsoft.CodeAnalysis.IPropertySymbol?
 Microsoft.CodeAnalysis.IPropertySymbol.PartialImplementationPart.get -> Microsoft.CodeAnalysis.IPropertySymbol?
 Microsoft.CodeAnalysis.IPropertySymbol.IsPartialDefinition.get -> bool

src/Compilers/Core/Portable/Symbols/INamespaceSymbol.cs

@@ -2,6 +2,7 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+using System;
 using System.Collections.Generic;
 using System.Collections.Immutable;
 using Microsoft.CodeAnalysis.Text;
@@ -27,6 +28,11 @@ public interface INamespaceSymbol : INamespaceOrTypeSymbol
         /// </summary>
         new IEnumerable<INamespaceOrTypeSymbol> GetMembers(string name);
 
+        /// <summary>
+        /// Get all the members of this symbol that have a particular name.
+        /// </summary>
+        void GetMembers<TArg>(string name, Action<INamespaceOrTypeSymbol, TArg> callback, TArg argument);
+
         /// <summary>
         /// Get all the members of this symbol that are namespaces.
         /// </summary>

src/Workspaces/SharedUtilitiesAndExtensions/Compiler/Core/SymbolKey/SymbolKey.NamespaceSymbolKey.cs

@@ -100,13 +100,16 @@ protected sealed override SymbolKeyResolution Resolve(
                         result.AddIfNotNull(module.GlobalNamespace);
                         break;
                     case INamespaceSymbol namespaceSymbol:
-                        foreach (var member in namespaceSymbol.GetMembers(metadataName))
-                        {
-                            if (member is INamespaceSymbol childNamespace)
+                        namespaceSymbol.GetMembers(
+                            metadataName,
+                            static (member, result) =>
                             {
-                                result.AddIfNotNull(childNamespace);
-                            }
-                        }
+                                if (member is INamespaceSymbol childNamespace)
+                                {
+                                    result.AddIfNotNull(childNamespace);
+                                }
+                            },
+                            result.Builder);
 
                         break;
                 }