Rework PooledHashSet<T> to avoid acquiring a HashSet<T> for 1 item

This change reworks PooledHashSet<T> and adds two features:

1. It is now possible to pass an IEqualityComparer<T> or a HashSetPool<T> when constructing a PooledHashSet<T>. A HashSetPool<T> will be chosen based on the IEqualityComparer<T>, or a new HashSet<T> will be created if a default pool doesn't exist.
2. A HashSet<T> won't be acquired from the pool (or created) until the set would contain at least two items.

Author: DustinCampbell

src/Compiler/Microsoft.CodeAnalysis.Razor.Compiler/src/CSharp/ComponentTagHelperDescriptorProvider.cs

@@ -125,7 +125,7 @@ private static TagHelperDescriptor CreateNameMatchingDescriptor(
             {
                 metadata.IsGeneric = true;
 
-                using var cascadeGenericTypeAttributes = new PooledHashSet<string>(SpecializedPools.StringHashSet.Ordinal);
+                using var cascadeGenericTypeAttributes = new PooledHashSet<string>(StringComparer.Ordinal);
 
                 foreach (var attribute in type.GetAttributes())
                 {
@@ -610,7 +610,7 @@ private static void CreateContextParameter(TagHelperDescriptorBuilder builder, s
         // - are not indexers
         private static ImmutableArray<(IPropertySymbol property, PropertyKind kind)> GetProperties(INamedTypeSymbol type)
         {
-            using var names = new PooledHashSet<string>(SpecializedPools.StringHashSet.Ordinal);
+            using var names = new PooledHashSet<string>(StringComparer.Ordinal);
             using var results = new PooledArrayBuilder<(IPropertySymbol, PropertyKind)>();
 
             var currentType = type;

src/Compiler/Microsoft.CodeAnalysis.Razor.Compiler/src/CSharp/DefaultTagHelperDescriptorFactory.cs

@@ -375,7 +375,7 @@ private static bool IsPotentialDictionaryProperty(IPropertySymbol property)
     private static void CollectAccessibleProperties(
         INamedTypeSymbol typeSymbol, ref PooledArrayBuilder<IPropertySymbol> properties)
     {
-        using var names = new PooledHashSet<string>(SpecializedPools.StringHashSet.Ordinal);
+        using var names = new PooledHashSet<string>(StringComparer.Ordinal);
 
         // Traverse the type hierarchy to find all accessible properties.
         var currentType = typeSymbol;