Align adaptor's generator accessors for attribute on the Op class

Each attribute has two accessor: one suffixed with `Attr` which returns the attribute itself
and one without the suffix which unwrap the attribute.
For example for a StringAttr attribute with a field named `kind`, we'll generate:

StringAttr getKindAttr();
StringRef getKind();

Differential Revision: https://reviews.llvm.org/D116466