[flang] Adapt mlir based error status handling in `tco` tool

Earlier scheme was using negative integers for communicating error status,
switch to canonical MLIR style.

f18 commit authored by @schweitz:
https://github.com/flang-compiler/f18-llvm-project/commit/c1af08d6e70b66c08ee2fbf2e63c764ec2e006e5

Reviewed By: mehdi_amini, clementval, svedanayagam

Differential Revision: https://reviews.llvm.org/D96068