Silence compiler warnings

mlir/lib/Parser/Parser.cpp:4484:15: warning: 'parseAssignmentList' overrides a member function but is not marked 'override' [-Winconsistent-missing-override]
  ParseResult parseAssignmentList(SmallVectorImpl<OperandType> &lhs,
              ^
mlir/include/mlir/IR/OpImplementation.h:662:3: note: overridden virtual function is here
  parseAssignmentList(SmallVectorImpl<OperandType> &lhs,
  ^
mlir/lib/Parser/Parser.cpp:4488:12: warning: unused variable 'type' [-Wunused-variable]
      Type type;
           ^