[flang] Add nsw flag to do-variable increment with a new option (#91579)

This patch adds nsw flag to the increment of do-variables when a new
option is enabled.
NOTE 11.10 in the Fortran 2018 standard says they never overflow.

See also the discussion in #74709 and the following discourse post.
https://discourse.llvm.org/t/rfc-add-nsw-flags-to-arithmetic-integer-operations-using-the-option-fno-wrapv/77584/5