[flang] Explanation of how to implement a semantic check

This is the story of implementing semantic checks for passing DO
variables to functions with dummy arguments with INTENT(OUT) or
INTENT(INOUT).

Original-commit: flang-compiler/f18@889f90913f280f285eb9960a0e0baadfd43a555f
Reviewed-on: https://github.com/flang-compiler/f18/pull/939