This commit defines macros F2018_LOCALITY and F2023_LOCALITY that
are used to swap code between the first two of what are now three
options for `do concurrent` locality:
1. Fortran 2018 locality: no reduce locality
2. Fortran 2023 locality: local and reduce locality
3. No locality.
where the compiler version determines which option is chosen.

A flang-new compiler error currently prevents use of a do-concurrent
local-spec.