Missing nullability warnings after invocation of extension methods with [AllowNull] this parameter

[TessenR]

Version Used:

Branch master (26 Sep 2020)
Latest commit 3e38d76 by CyrusNajmabadi:
Merge pull request #48002 from CyrusNajmabadi/implicitCreation

Use implicit object creation in the IDE layers.

Steps to Reproduce:

Compile and run the following code:

#nullable enable
using System.Diagnostics.CodeAnalysis;

class Test
{
  public static void Main()
  {
      string? s = null;
      s.AllowNullExtension();
      s.ToString(); // no warnings, runtime crash
  }
}

public static class Extension
{
   public static void AllowNullExtension([AllowNull] this string p) { }
}

Expected Behavior:
Warning for s.ToString()

Actual Behavior:
No warnings. The program crashes at runtime with a NullReferenceException

[PathogenDavid]

Worth noting that this is not specific to extension methods, it happens with normal methods too.

[RikkiGibson]

This appears to be a corollary of #48605 and would likely be solved at the same time