[AsmDiff] Nullability attributes are not being detected properly

[carlossanlop]

Found on this comment on the Preview1 api-diff PR: https://github.com/dotnet/core/pull/7211/files#r810072025

Here is how the API looks in Preview1: https://github.com/dotnet/runtime/blob/405337939cf8d793661f6c6c0007c3f519b26f0d/src/libraries/Microsoft.Extensions.FileSystemGlobbing/ref/Microsoft.Extensions.FileSystemGlobbing.cs#L153

AsmDiff generated it like this:

-        public override bool Equals(object obj);
+        public override bool Equals(object? obj);

Also a problem here: https://github.com/dotnet/core/pull/7211/files#r810086046

And here: https://github.com/dotnet/core/pull/7211/files#r810192792