Reduce allocations in TextDocumentStates.SelectAs* methods (#72159)

These methods were using an unsized ArrayBuilder to create an ImmutableArray.

Instead, as the size is pre-known in all these methods, we can just use ImmutableArray.CreateBuilder directly.

src/Workspaces/Core/Portable/Workspace/Solution/TextDocumentStates.cs

@@ -101,38 +101,41 @@ public IEnumerable<TState> GetStatesInCompilationOrder()
 
         public ImmutableArray<TValue> SelectAsArray<TValue>(Func<TState, TValue> selector)
         {
-            using var _ = ArrayBuilder<TValue>.GetInstance(out var builder);
+            // Directly use ImmutableArray.Builder as we know the final size
+            var builder = ImmutableArray.CreateBuilder<TValue>(_map.Count);
 
             foreach (var (_, state) in _map)
             {
                 builder.Add(selector(state));
             }
 
-            return builder.ToImmutable();
+            return builder.MoveToImmutable();
         }
 
         public ImmutableArray<TValue> SelectAsArray<TValue, TArg>(Func<TState, TArg, TValue> selector, TArg arg)
         {
-            using var _ = ArrayBuilder<TValue>.GetInstance(out var builder);
+            // Directly use ImmutableArray.Builder as we know the final size
+            var builder = ImmutableArray.CreateBuilder<TValue>(_map.Count);
 
             foreach (var (_, state) in _map)
             {
                 builder.Add(selector(state, arg));
             }
 
-            return builder.ToImmutable();
+            return builder.MoveToImmutable();
         }
 
         public async ValueTask<ImmutableArray<TValue>> SelectAsArrayAsync<TValue, TArg>(Func<TState, TArg, CancellationToken, ValueTask<TValue>> selector, TArg arg, CancellationToken cancellationToken)
         {
-            using var _ = ArrayBuilder<TValue>.GetInstance(out var builder);
+            // Directly use ImmutableArray.Builder as we know the final size
+            var builder = ImmutableArray.CreateBuilder<TValue>(_map.Count);
 
             foreach (var (_, state) in _map)
             {
                 builder.Add(await selector(state, arg, cancellationToken).ConfigureAwait(true));
             }
 
-            return builder.ToImmutable();
+            return builder.MoveToImmutable();
         }
 
         public TextDocumentStates<TState> AddRange(ImmutableArray<TState> states)