[mlir][nfc] Fully spell mlir typename in generated code.

mlir/tools/mlir-tblgen/OpFormatGen.cpp

if (auto *attr = dyn_cast<AttributeVariable>(element)) {
     auto *typeOperand = dir->getArg();
     auto *operand = dyn_cast<OperandVariable>(typeOperand);
     auto *var = operand ? operand->getVar()
                         : cast<ResultVariable>(typeOperand)->getVar();
     std::string name = op.getGetterName(var->name);
     if (var->isVariadic())
       body << name << "().getTypes()";
     else if (var->isOptional())
-      body << llvm::formatv("({0}() ? {0}().getType() : Type())", name);
+      body << llvm::formatv("({0}() ? {0}().getType() : ::mlir::Type())", name);
     else
       body << name << "().getType()";
 
   } else if (auto *string = dyn_cast<StringElement>(element)) {
     FmtContext ctx;
     ctx.withBuilder("::mlir::Builder(getContext())");
     ctx.addSubst("_ctxt", "getContext()");
     body << tgfmt(string->getValue(), &ctx);