Better support for attribute wrapper classes when getting def name

Unless we explicitly name a template instantiation in .td file, its def
name will be "anonymous_<number>". We typically give base-level Attr
template instantiation a name by writing `def AnAttr : Attr<...>`. But
when `AnAttr` is further wrapped in classes like OptionalAttr, the name
is lost unless explicitly def'ed again. These implicit-named template
instantiation is fairly common when writing op definitions. Those wrapper
classes are just essentially attaching more information to the attribute.
Without a proper way to trace back to the original attribute def name
can cause problems for consumers wanting to handle attributes according
to their types.

Previously we handled OptionalAttr and DefaultValuedAttr specifically,
but Confined was not supported. And they can compose together to have
Confined<OptionalAttr<...>, [...]>. So this CL moves the baseAttr field
to main Attr class (like isOptional) and set it only on the innermost
wrapper class.

PiperOrigin-RevId: 258341646