Warn when a synchronized loop never ends

[GerardSmit]

This PR detects never ending loops after the method was changed to synchronous code.

Example

From the file WhileNotCancelled.cs in GenerationSandbox.Tests:

using System;
using System.Threading;
using System.Threading.Tasks;

namespace GenerationSandbox.Tests;

internal static partial class WhileNotCancelled
{
    [Zomp.SyncMethodGenerator.CreateSyncVersion]
    public static async ValueTask SleepAsync(CancellationToken ct)
    {
        while (!ct.IsCancellationRequested)
        {
            await Task.Delay(120000, ct);
        }

        throw new OperationCanceledException();
    }
}

Before

The synchronized version of this code will never end, because !ct.IsCancellationRequested gets replaced with true. The last statement (throw) will also show an error that it'll never be reached:

After

In the new version the last statement gets removed:

public static void Sleep()
{
    while (true)
    {
        global::System.Threading.Thread.Sleep(120000);
    }
}

And a new diagnostic is added (ZSMGEN004) telling the user that it's detected that the loop will never end:

WhileNotCancelled.cs(12,9): Warning ZSMGEN004 : It is detected that the while loop will never end after transforming to synchronous version

Please validate if the error message is correct or should be changed:

1656a54#diff-59a20cb680fe0e33b6def06d74d7c85b3e090d4c9b716668e145f74715af9fecR29-R35

[virzak]

Rebased onto master and reworked on top: the warning is now ZSMGEN006 (004 is taken), judged on the rewritten loop body, and it no longer fires for hand-written while (true) or once #if SYNC_ONLY replaces the loop.