[flang][NFC] Move Todo.h from Lower to Optimizer

Remove a backwards dependence from Optimizer -> Lower by moving Todo.h
to the optimizer and out of lowering.

This patch is part of the upstreaming effort from fir-dev branch.

Co-authored-by: Eric Schweitz <eschweitz@nvidia.com>

Reviewed By: jeanPerier

Differential Revision: https://reviews.llvm.org/D127292