[Perf] Pre-allocate array builder sizes for loading PE members

src/Compilers/Core/Portable/MetadataReader/PEModule.cs

@@ -627,7 +627,7 @@ private IEnumerable<TypeDefToNamespace> GetTypeDefsOrThrow(bool topLevelOnly)
             GetTypeNamespaceNamesOrThrow(namespaces);
             GetForwardedTypeNamespaceNamesOrThrow(namespaces);
 
-            var result = new ArrayBuilder<IGrouping<string, TypeDefinitionHandle>>();
+            var result = new ArrayBuilder<IGrouping<string, TypeDefinitionHandle>>(namespaces.Count);
 
             foreach (var pair in namespaces)
             {