Refs pointing to differently annotated variables / fields creates dataflow hole

The linker has analysis holes around ref fields that easily allows annotated pointers to point to differently annotated variables / fields. This means a ref field with PublicMethods could point to a field with PublicMethods and PublicFields, then assign a value with PublicMethods but not PublicFields to the location.

Another example:

```C#
static void AnalysisHole<
T,
[DAMT.PublicMethods)] TM,
[DAM (DAMT.PublicMethods | DAMT.PublicFields)] TMF
>
(scoped RefFieldWithMethods target,
[DAMT.PublicMethods | DAMT.PublicFields)] Type tmf)
{
var tmLocal = typeof (TM);
target.T = ref tmLocal;
tmLocal = typeof (T); // Should warn for assigning and unannotated value to a location pointed to by an annotated pointer

target.T = ref tmf;
target.T = typeof (TM); // Should warn for assigning a value with only Methods to a location annotated with Methods and Fields
}

ref struct RefFieldWithMethods
{
[DynamicallyAccessedMembers (DAMT.PublicMethods)]
public ref Type T;
}
```

Related to https://github.com/dotnet/linker/issues/2158