Version Used: 11.0.100-alpha.1.26060.102

Steps to Reproduce:

1. Remove pragma warning disable from https://github.com/dotnet/dotnet/blob/65edeb2ddb6cc6419f10c520712026f5d0813921/src/winforms/src/Common/tests/TestUtilities/TestAccessors.cs#L15-L20
2. Build the VMR / winforms
3. Observe the IDE0052 warning / error

Diagnostic Id: IDE0052 (Remove unread private member)

Expected Behavior:
IDE0052 not raised as the s_nullObjectParam field is read in the extension in the same file.

Actual Behavior:
IDE0052 raised as the analyzer probably doesn't take the extension in the same file into account.

cc @jcouv @CyrusNajmabadi