Analyzer proposal: pass CancellationToken explicitly

[sharwell]

This analyzer would report cases where a method is called with an implicit cancellation token:

async Task Method1Async() {
  await Method2(); // Diagnostic
  await Method3(); // Diagnostic
  Method4(); // Diagnostic
}

async Task Method2Async(CancellationToken cancellationToken = default) { }

async Task Method3Async() { }
async Task Method3Async(CancellationToken cancellationToken) { }

void Method4() { }
void Method4(CancellationToken cancellationToken) { }

Each of the three cases [should|should not] have the same diagnostic ID:

1. Calling an asynchronous method using the default value for an optional CancellationToken parameter
2. Calling an overload of an asynchronous method, where a different overload accepts a CancellationToken
3. Calling a synchronous method that accepts a CancellationToken (as an optional parameter or a different overload)

[AArnott]

See also #394 and consider how they will relate.

[gundermanc]

+1 on this one. See this recent regression bug in LSP client due to failure to propagate CancellationTokens: https://devdiv.visualstudio.com/DevDiv/_workitems/edit/1083283

It has actually been fixed on at least 2 occasions before.

[Youssef1313]

This was implemented in roslyn-analyzers in dotnet/roslyn-analyzers#3641