[BoundsSafety] Move `FieldDecl::IsBoundsSafetyCounter` to an attribute.

[rapidsna]

IsBoundsSafetyCounter is used to indicate that the field is referred to by another field that has __counted_by or friends. __counted_by will be extended to be used on a variable. Hence, we would need an attribute to apply to a non-field decl and hold more information about the other variable referring to it.

[bwendling]

I removed this flag.