Adjust pattern-based lookup for `await foreach`

[jcouv]

Pattern-based 'await foreach' should find 'GetAsyncEnumerator' and 'MoveNextAsync' by binding them with no arguments.

So GetAsyncEnumerator(params int[] x) or GetAsyncEnumerator(int optional = 0) would be recognized, in addition to GetAsyncEnumerator() and GetAsyncEnumerator(CancellationToken token = default).