Fix directive processing

[GerardSmit]

Before the directives weren't processed, which is causing problems.

For example, I have the following code:

using System;
namespace Test;
public partial class DisposeTest
{
    [CreateSyncVersion]
    public async Task DisposeAsync(object? obj)
    {
#if !SYNC_ONLY
            if (obj is IAsyncDisposable asyncDisposable)
            {
                await asyncDisposable.DisposeAsync();
                return;
            }
#endif

            if (obj is IDisposable disposable)
            {
                disposable.Dispose();
                return;
            }

#if SYNC_ONLY
            if (obj is IAsyncDisposable)
            {
                throw new InvalidOperationException("Cannot dispose an async disposable in sync-only mode.");
            }
#endif
}

In the sync result, the using statements are removed, causing if (obj is IAsyncDisposable) to fail (the namespace isn't being added, because we are not processing the directives).

This PR fixes this. The directives are being processed, causing the namespace to be added.

Diffs (before, after):

//HintName: Test.DisposeTest.DisposeAsync.g.cs
// <auto-generated/>
#nullable enable
namespace Test;
public partial class DisposeTest
{
    public void Dispose(object? obj)
    {

            if (obj is global::System.IDisposable disposable)
            {
                disposable.Dispose();
                return;
            }

-           if (obj is IAsyncDisposable)
+           if (obj is global::System.IAsyncDisposable)
            {
-               throw new InvalidOperationException("Cannot dispose an async disposable in sync-only mode.");
+               throw new global::System.InvalidOperationException("Cannot dispose an async disposable in sync-only mode.");
            }
}
}