[release/dev18.3] Extensions: Optimize GetExtensionMembers (#82153)

Backport of #82124 to release/dev18.3

Author: github-actions[bot]

src/Compilers/CSharp/Portable/Symbols/NamedTypeSymbol.cs

@@ -420,7 +420,7 @@ internal void GetExtensionMembers(ArrayBuilder<Symbol> members, string? name, st
 
             if (!this.IsClassType() || !IsStatic || IsGenericType || !MightContainExtensionMethods) return;
 
-            foreach (NamedTypeSymbol nestedType in GetTypeMembersUnordered())
+            foreach (NamedTypeSymbol nestedType in GetTypeMembers(name: ""))
             {
                 if (nestedType is not { IsExtension: true, ExtensionParameter: { } extensionParameter }
                     || !IsValidExtensionReceiverParameter(extensionParameter))

src/Compilers/CSharp/Portable/Symbols/Retargeting/RetargetingNamespaceSymbol.cs

@@ -36,6 +36,11 @@ internal sealed class RetargetingNamespaceSymbol
         /// </summary>
         private readonly NamespaceSymbol _underlyingNamespace;
 
+        /// <summary>
+        /// Cache of the type members.
+        /// </summary>
+        private ImmutableArray<NamedTypeSymbol> _lazyTypeMembers;
+
         public RetargetingNamespaceSymbol(RetargetingModuleSymbol retargetingModule, NamespaceSymbol underlyingNamespace)
         {
             Debug.Assert((object)retargetingModule != null);
@@ -105,12 +110,18 @@ public override ImmutableArray<Symbol> GetMembers(ReadOnlyMemory<char> name)
 
         internal override ImmutableArray<NamedTypeSymbol> GetTypeMembersUnordered()
         {
-            return RetargetTypeMembers(_underlyingNamespace.GetTypeMembersUnordered());
+            return GetTypeMembers().ConditionallyDeOrder();
         }
 
         public override ImmutableArray<NamedTypeSymbol> GetTypeMembers()
         {
-            return RetargetTypeMembers(_underlyingNamespace.GetTypeMembers());
+            if (_lazyTypeMembers.IsDefault)
+            {
+                var typeMembers = RetargetTypeMembers(_underlyingNamespace.GetTypeMembers());
+                ImmutableInterlocked.InterlockedInitialize(ref _lazyTypeMembers, typeMembers);
+            }
+
+            return _lazyTypeMembers;
         }
 
         private ImmutableArray<NamedTypeSymbol> RetargetTypeMembers(ImmutableArray<NamedTypeSymbol> underlyingMembers)