[mlir] Add Adaptor alias

[jpienaar]

Summary

Add Adaptor alias alongside OperandAdaptor to make next renaming more
mechanical. OperandAdaptor's are no longer just about operands.
Considered OpAdaptor too, but then noticed we'd mostly end up with
XOp::OpAdaptor which seems redundant.

[ftynse]

Note that we also have mlir::OperandAdaptor in IR/OperationSupport.h

mlir/lib/TableGen/OpClass.cpp

 
 void tblgen::OpClass::writeDeclTo(raw_ostream &os) const {
   os << "class " << className << " : public Op<" << className;
   for (const auto &trait : traitsVec)
     os << ", " << trait;
   os << "> {\npublic:\n";
   os << "  using Op::Op;\n";
   os << "  using OperandAdaptor = " << className << "OperandAdaptor;\n";
+  os << "  using Adaptor = " << className << "OperandAdaptor;\n";
 
   bool hasPrivateMethod = false;
   for (const auto &method : methods) {
     if (!method.isPrivate()) {
       method.writeDeclTo(os);
       os << "\n";
     } else {
       hasPrivateMethod = true;